Otto Eugster

Cosmic-ray production rates for 3He, 21Ne, 38Ar, 83Kr, and 126Xe in chondrites based on 81Kr-Kr exposure ages

81Kr-Kr exposure ages and the isotopic abundances of He, Ne, Ar, Kr, and Xe were measured for thirteen ordinary chondrites. Together with the 81Kr-Kr ages for four chondrites obtained earlier in our laboratory and two chondrites reported by other authors, this data base is used to derive cosmic-ray production rates for 3He, 21Ne, 38Ar, 83Kr, and 126Xe. Production rates are obtained as a function of the shielding dependent cosmogenic ratio (22Ne21Ne)c and of the chemical composition of CI, CM, CO, CV, H, L, LL, EH, and EL chondrites. The following production rate equations are proposed (P3),P21, and P38in units of 10−8 cm3 STP/g per Ma, P83and P126 in 10−12 cm3 STP/g per Ma): P3 = F[2.09 − 0.43(22Ne21Ne)c], FCI = 1.01, FCM = 1.00, FCO = 0.99, FCV = 0.99, FH = 0.98 , FL=1.00, FLL=1.00, Feh = 0.97, and FEL = 1.00 . P21 = 1.61F[21.77(22Ne21Ne)c− 19.32]−1, FCI = 0.67, FCM = 0.79, FCO = 0.96, FCV = 0.96, FH = 0.93 , FL=1.00, FLL=1.00, FEH = 0.78, and FEL = 0.96 . P38 = F[0.125 − 0.071(22Ne21Ne]c, FCI, = 0.75, FCM = 0.88, FCO = 1.03, FCV = 1.10, FH = 1.08 , FL=1.00, FLL=1.00, FEH = 0.98, and FEL = 0.89 . P83 = 0.0196F[0.62(22Ne21Ne)c − 0.53]−1, FCI = 0.71, FCM = 0.94,ifCO = 1.02, FCV = 1.13, FH = 1.00 , FL=1.00, FLL=1.00, FEH = 0.75, and FEL = 0.80 . P126 = F[0.0174 − 0.0094(22Ne21Ne)c], FCI = 0.66, FCM = 0.93, FCO = 1.18, FCV = 1.40, FH = 1.00 , FL = 1.00, FLL = 1.00, FEH = 0.72, and FEL = 0.72 . Exposure ages were calculated from cosmogenic 3He, 21Ne, and 38Ar concentrations in eleven chondrites which lack Kr data. The good agreement of the ages derived from different noble gases demonstrates the consistency of the adopted production rates. Finally, noble gas data from mineral separates and from stepped temperature fractions show that Kr and Xe extracted between 700 and 1200°C from a feldspar-rich separate have large excesses of the cosmogenic component. This should result in more accurate 81Kr-Kr and 126Xe ages than bulk chondrite analyses.

New W-isotope evidence for rapid terrestrial accretion and very early core formation

The short-lived Hf-182-W-182-isotope system is an ideal clock to trace core formation and accretion processes of planets. Planetary accretion and metal/silicate fractionation chronologies are calculated relative to the chondritic Hf-182-W-182-isotope evolution. Here, we report new high-precision W-isotope data for the carbonaceous chondrite Allende that are much less radiogenic than previously reported and are in good agreement with published internal Hf-W chronometry of enstatite chondrites. If the W-isotope composition of terrestrial rocks, representing the bulk silicate Earth, is homogeneous and 2.24 epsilon(182W) units more radiogenic than that of the bulk Earth, metal/silicate differentiation of the Earth occurred very early. The new W-isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion. Early terrestrial core formation implies rapid terrestrial accretion, thus permitting formation of the Moon by giant impact while Hf-182 was still alive. This could explain why lunar W-isotopes are more radiogenic than the terrestrial value. Copyright (C) 2002 Elsevier Science Ltd.

Krypton and xenon isotopic composition in three carbonaceous chondrites

The concentrations and isotopic composition of Kr and Xe have been measured in the three carbonaceous chondrites Cold Bokkeveld, Lance and Orgueil. Spallation corrections and, in the case of Cold Bokkeveld also neutron effects, are negligible and for the average Kr and Xe isotopic composition in carbonaceous chondrites we obtain: δ7886=−(3.4±0.8)% δ8086=−(2.4±0.8)% δ8286=−(1.7±0.4)% δ8386=−(1.35±0.35)% δ8486=−(1.35±0.35)% and δ124130=+(21.0±3.0)% δ126130=+(16.0±1.5)% δ128130=+(8.3±1.0)% δ131130=−(2.3±0.8)% δ132130=−(5.8±0.7)% δ134130=−(7.3±0.8)% δ136130=−(8.3±0.8)% The significance and the possible origin of these anomalies are briefly discussed.

Formation of Metal and Silicate Globules in Gujba: A New Bencubbin-like Meteorite Fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ15N values (∼+685‰). The 3He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.

Cosmic rays, carbon dioxide, and climate

Several recent papers have applied correlation analysis to climate-related time series in the hope of finding evidence for causal relationships. For a critical discussion of correlations between solar variability, cosmic rays, and cloud cover, see Laut [2003]. A prominent new example is a paper by Shaviv and Veizer [2003], which claims that fluctuations in cosmic ray flux reaching the Earth can explain 66% of the temperature variance over the past 520 m.y.,and that the sensitivity of climate to a doubling of CO2 is less than previously estimated.

The isotopic composition of krypton in unequilibrated and gas rich chondrites

Abstract After correcting for cosmic-ray produced spallation and (n, γ) components the isotopic composition of Kr in the investigated 3 unequilibrated and 2 gas-rich chondrites is similar. The average δ-values relative to atmospheric Kr in percent are: δ 86 78 = −4.05 ± 1.1 ; δ 86 80 = −1.25 ± 0.8 ; δ 86 82 = −2.05 ± 0.5 ; δ 86 83 = −1.55 ± 0.5 ; δ 86 84 = −1.6 ± 0.4 . The low 78 Kr abundance appears to exclude the possibility that an addition of fission Kr alone is responsible for the observed differences. Mass fractionation, together with some Kr from neutron capture on Br, could just within the experimental uncertainties explain the results. A model combining mass fractionation and the addition of fission Kr would, however, explain the experimental data much better. Some other processes which could have influenced the terrestrial or meteoritic Kr isotopic composition are also discussed, but their contributions are likely to be negligible.

History of Meteorites from the Moon Collected in Antarctica

In large asteroidal or cometary impacts on the moon, lunar surface material can be ejected with escape velocities. A few of these rocks were captured by Earth and were recently collected on the Antarctic ice. The records of noble gas isotopes and of cosmic ray—produced radionuclides in five of these meteorites reveal that they originated from at least two different impact craters on the moon. The chemical composition indicates that the impact sites were probably far from the Apollo and Luna landing sites. The duration of the moon-Earth transfer for three meteorites, which belong to the same fall event on Earth, lasted 5 to 11 million years, in contrast to a duration of less than 300,000 years for the two other meteorites. From the activities of cosmic ray—produced radionuclides, the date of fall onto the Antarctic ice sheet is calculated as 70,000 to 170,000 years ago.

81Kr in meteorites and81Kr radiation ages

Abstract The 2.13 × 105 y81Kr was determined mass-spectrometrically in gram-sized samples of five stone meteorites. The following concentrations were found (in units of 10−15 cm3 STP/g): Stannern: 205 ± 20; Khor Temiki: 10 ± 3; Mocs: 31 ± 7; Ochansk: 19 ± 6; Parnallee: 27 ± 4. From the measured isotopic composition of krypton78Kr/81Kr)spall ratios and radiation ages were derived. No assumptions on shielding and chemical composition have to be made and81Kr radiation ages are therefore inherently more accurate than the ages obtained from the concentrations of stable spallation isotopes alone.

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

Abstract 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.

Noble gas isotopic composition, cosmic ray exposure history, and terrestrial age of the meteorite Allan Hills A81005 from the moon

We present a comprehensive study of the elemental and isotopic abundances of the noble gases He, Ne, Ar, Kr, and Xe in the meteorite Allan Hills A81005 from the Moon. In addition to a bulk sample five grain size fractions were analyzed. Chemical abundances relevant for the interpretation of the cosmic-ray-produced noble gases were determined and indicate that the grain size fractions are chemically uniform. Except for the fact that the trapped noble gas concentrations appear to be grain size correlated, the isotopic and elemental pattern of the trapped solar wind noble gases in A81005 are very similar to those observed in lunar soils and breccias. The A81005 material resided during (580 ± 180) m.y. in the nuclear active zone of the lunar regolith at an average shielding depth of about 40 g/cm2. From literature data we conclude that the Moon-Earth transit time lasted less than a few million years. Finally, A81005 was captured by the Earth more than 140,000 years ago as obtained from the abundance of cosmic-ray-produced81Kr.