Hans J. Hofmann

Depth and size dependence of cosmogenic nuclide production rates in stony meteoroids

Abstract Depth profiles of the cosmogenic isotopes 3He, 20e, 21Ne, 22Ne, 10Be and 26A1 have been measured by conventional and accelerator mass spectrometry in the chondrites Madhipura, Udaipur, and Bansur. Shielding depths of the samples and meteorite sizes were derived from cosmic ray track density data and from 21Ne exposure ages. In addition, 10Be and 26A1 were measured in seven fragments of Dhajala. The measured data, together with the existing 53Mn profiles in these meteorites and with other well-investigated depth profiles of cosmogenic radionuclides and rare gas isotopes in ALHA 78084, Keyes, St. Severin, Jilin and Knyahinya, now provide an experimental data base describing the depth and size dependence of cosmogenic nuclides in ordinary chondrites for preatmospheric radii between 8.5 cm and about 100 cm. Production rates are found to change only slightly with depth in small meteorites (R ≤ 15 cm). For larger bodies (15 cm ≤ R ≤ 65 cm), the profiles show significant depth dependence, the cosmogenic production increases from the surface to the center by about 30%. The center production rates increase with meteoroid size and show a broad maximum for radii between 25 and 65 cm. The location of the maxima for different nuclides depends on the dominant energy of particles responsible for their production from the main target elements. For R ≥ 70 cm, a significant decrease of center production rates is seen for 10Be, 26A1,53Mn and 21Ne, the individual depth profiles being essentially flat with shallow transition maxima. The observed depth profiles and the dependence of the center production rates on meteoroid size are well reproduced by model calculations based on Monte Carlo calculations of the intra- and internuclear cascade of galactic protons in meteoritic matter and on experimental and theoretical excitation functions of the underlying nuclear reactions. The model calculations provide a basis for identification of meteorites with anomalous levels of radioisotopes and give information about the irradiation history of meteorites and changes in the cosmic ray intensity with time and orbital space of the meteoroid. The results of the Dhajala chondrite are discussed in this context.

Production of residual nuclei by proton-induced reactions on C, N, O, Mg, AI and Si

Cross sections for the production of residual nuclides by p-induced reactions are the basic nuclear quantities for an accurate modelling of the interaction of solar cosmic protons with matter. In a series of irradiation experiments at the cyclotron of the Svedberg Laboratory/University of Uppsala sixteen different target elements were investigated for proton energies up to 100 MeV in order to determine such thin-target excitation functions. Residual nuclides were measured by gamma-spectrometry and, in the case of10Be and26Al, by accelerator mass spectrometry. Here, we report results for the light target elements C, N, O, Mg, Al, and Si including also new cross sections for the production of10Be and26Al from aluminum for p-energies between 100 MeV and 200 MeV. The latter were derived from targets irradiated earlier at the IPN Orsay. The new experimental data are compared with earlier work and analyzed in the framework of the hybrid model of preequilibrium reactions investigating several options with respect to the choice of nuclear masses including also corrections for shell effects of nuclear masses and level densities.

Mass-spectrometric 10Be dating of deep-sea sediments applying the Zürich tandem accelerator

Abstract We have measured 10 Be in deep-sea sediment cores from the Central North Pacific (GPC-3, 30° N 157° W) and from areas with high biological activity, off West Africa and the South Atlantic near Antarctica. In core GPC-3 we observe a decrease with depth which can be decomposed into three exponentials. Sediment accumulation rates of 2 mm/ka for the upper 2.2 m, 1.1 mm/ka for the section between 2.2 m and 4.7 m and 0.5 mm/ka from 4.7 m to 10 m have been derived. The changes in the accumulation rate occurred at 1.1 Ma and 3.3 Ma BP. In core GPC-3 the 10 Be chronology agrees (20%) with paleomagnetic dating of the uppermost 4.2 m, corresponding to a time range of 2.43 Ma (Matuyama-Gauss Boundary). The average 10 Be flux into the sediments at locality GPC-3 during the past 1.1 Ma is comparable to the assumed depositional flux from the atmosphere. On sediment cores from off West Africa and the South Atlantic we observe, on the contrary, fluxes of 10 Be which exceed more than 10 times the depositional flux. This indicates that the concentration of 10 Be in the open ocean at localities in the proximity of highly productive areas may be strongly modulated by variations of bioactivity throughout time.

History of the paired lunar meteorites MAC88104 and MAC88105 derived from noble gas isotopes, radionuclides, and some chemical abundances☆

Abstract Noble gas isotopes, radionuclides, and chemical abundances were studied in the lunar meteorites MAC88104 and MAC88105 collected in the MacAlpine Hills area of Antarctica. The concentrations of the noble gas isotopes and the radionuclide activities in the two meteorites are essentially identical, proving that the two meteorites are paired. From 40K-40Ar dating we obtain a gas retention age of 3550 ± 400 Ma, typical for lunar surface material. Probably before breccia compaction the MAC88104/5 material resided for 630 ± 200 Ma at an average shielding depth of 85 g cm 2 , that is, about 50 cm below the lunar surface in the lunar regolith, as judged from the concentration of cosmic-ray produced Kr and Xe isotopes. Although this duration of lunar regolith residence is relatively long, MAC88104/5 represent immature regolith material: the concentration of solar wind implanted noble gases are two orders of magnitude lower than those in mature lunar soil. The reason for that might be the relatively high average shielding. The 40 Ar 36Ar ratio of the trapped component is 5.7 ± 1.0, indicating an intermediate antiquity of the material; we estimate that the solar wind and lunar atmospheric particles were implanted about 2000 Ma ago. The radionuclide activities allow us to derive the exposure history of the MAC88104/5 material. The duration of the Moon-Earth transfer was ≦0.24 Ma. Production of the radionuclides on the lunar surface immediately before ejection is possible, but duration and shielding depth for such an exposure cannot be determined from our data. We obtain a terrestrial age of 0.1 ≦ tterr ≦ 0.6 Ma and an ejection time of MAC88104/5 from the Moon of 0.1 ≦ tejection ≦ 0.8 Ma. The exposure histories of the lunar meteorites discussed in this work indicate that at least two impact events are required for their ejection from the Moon. Our first noble gas results for lunar meteorite Yamato-793274 show that it represents mature lunar regolith material with relatively high concentrations of solar wind implanted noble gases and a duration of several hundred million years of exposure to cosmic rays.

Diffusion and supply rates of 10Be and 230Th radioisotopes in two manganese encrustations from the South China Sea

Ages of two Mn encrustations estimated by their 230Th and 10Be distributions are compared with K-Ar ages and micropaleontological datings of their nuclei to discuss possible diffusion and supply effects of the radioisotope distribution and their influence on the reliability of age determinations. Based on comparable results obtained by the different methods the effective diffusion coefficient of 10Be can be calculated as D∗ ≤ 1.0·10−8cm2/y. This coefficient is 3–8 times smaller than the best estimates available at present. In both nodules we observe lower 10Be concentrations in the uppermost 2–3 mm (1.3 m.y.), which suggests that 10Be uptake has been reduced since the middle Pleistocene. The 2.7-fold increase of the growth rate starting 3.2 m.y. ago coincides with the initiation of the northern hemisphere glaciation.

AMS measurements of thin-target cross sections for the production of 10Be and 26 Al by high-energy protons

Abstract In order to determine thin-target cross sections for the proton-induced prduction of residual nuclides from cosmochemically relevant target elements (Z≤28), irradiation experiments were carried out with energies between 100 and 2600 MeV at various accelerators. For p-energies below 200 MeV the stacked-foil technique was used, while for higher energies individual thin targets were irradiated at 600, 800, 1200 and 2600 MeV. Using accelerator mass spectrometry, cross sections for the production of Be from O, Mg, Al, Si, Mn, Fe and Ni and of 26Al from Al, Mn, Fe and Ni were measured. The experimental data are compared with earlier literature data and the present status of the respective excitation functions is discussed. Theoretical cross sections for the reactions investigated were derived by Monte Carlo calculations on the basis of an intranuclear-cascade/evaporation (INC/E) model using the HERMES code system. For energies below 200 MeV, hybrid model calculations were performed using the code Alice Livermore 87. The calculated and experimental data are compared and the different production modes of 10Be and 26Al are discussed.

10Be dating of the inner structure of Mn-encrustations applying the Zürich tandem accelerator

Abstract We have measured 10 Be and 230 Th depth profiles in Mn-nodules and crusts from the Central North Pacific (2 samples), from the South China Sea (2 samples) and from the South Pacific (1 sample). Comparison of the nucleis ages derived via 230 Th and 10 Be datings and other methods (K/Ar and paleontology) shows very good agreement and proofs that distortion of 10 Be and 230 Th ages due to radionuclide diffusion in Mn-encrustations is negligible ( In all samples we observe a smooth exponential decrease for larger sections, suggesting constant depositional 10 Be fluxes within the time resolution of 0.4 to 0.8 Ma. (resulting from the thickness of the sampled intervals). Growth rates range from 2 mm/Ma to up to 20 mm/Ma. Common to all samples is a remarkable change of growth rate accompanied by a visible change of the mineralogical inner texture, dated at 6.2 Ma. We observe slower growth rates during the last 6.2 Ma than in the previous periods. Provided that the Mn-concentration in deep water has remained constant throughout time (as supported by nearly constant trace metal/manganese ratios of the samples) the data suggest that the bottom water circulation pattern has slowed down since 6.2 Ma BP. Visible changes in the inner texture corresponding to paleooceanographic time marks such as 1.3 Ma (a presumed variation of the ice volume during Middle Pleistocene), 3.3 Ma (Glaciation of the Northern Hemisphere), and 12–14 Ma ago (maximum Glaciation of the Antarctica) strongly suggest that the growth pattern of Mn-encrustations has been influenced by the response of the oceans to the history of climatic changes.

Cosmogenic nuclides in differentiated antarctic meteorites: measurements and model calculations

Abstract The cosmogenic radionuclides 10Be, 26Al and 53Mn and stable He-, Ne- and Ar-isotopes were measured in differentiated meteorites from Antarctica using accelerator mass spectrometry, γ-γ-coincidence techniques, radiochemical neutron activation analysis and conventional mass spectrometry. No depth effects were seen in meteorites from which several samples were analyzed. In most of the meteorites 10Be and 26Al were in saturation at time of fall, but the 26Al concentrations are partially lowered by substantial terrestrial ages. For 10Be some extremely low concentrations were found which cannot be explained by the decay during terrestrial residence. The experimental data are discussed together with rare gas measurements in the context of model calculations of the depth- and size-dependent production of cosmogenic nuclides in differentiated stony meteorites. Based on the model calculations minimal exposure ages, 21 Ne 53 Mn and shielding corrected 21Ne exposure ages were calculated. A detailed discussion of the production rates and of possible pairing of meteorites is given.

Cosmogenic nuclides in eucrites

Abstract The cosmogenic nuclides 10 Be and 26 Al were measured in a number of selected Antarctic and non-Antarctic eucrites by accelerator mass spectrometry. Whenever available, different samples from the same meteorite were analyzed, in order to get information on depth-dependent variations of activities of cosmogenic radionuclides. For 26 Al, measurements by gamma-gammacoincidence techniques were also done. For the Antarctic meteorites stable He-, Ne- and Ar-isotopes were determined in the same samples by conventional mass spectrometry. A detailed mineralogical study of the meteorites analyzed allowed a classification or reclassification as monomict and polymict eucrites. In meteorite falls 10 Be and 26 Al are in saturation. Some 10 Be and most 26 Al data in Antarctic eucrites are lower than those in falls. For 26 Al this is attributed to the long terrestrial residence times of Antarctic meteorites. For 10 Be the lower concentrations are unlikely to be due to the decay during terrestrial residence. The experimental data are discussed together with rare gas measurements in the context of model calculations of the depth- and size-dependent production of cosmogenic nuclides in eucrites. In the case of four Antarctic Allan Hills eucrites a possible pairing is derived on the basis of cosmogenic radionuclides, stable rare gas isotopes and mineralogical similarity.

Proton-Induced Spallation Between 600 and 2600 MeV

In the course of a systematic investigation of proton-induced reactions fifteen target elements ranging from O to Au were irradiated with protons having energies of 600, 800, 1200, 1600 and 2600 MeV. Thin-target cross sections for the production of residual radionuclides were measured by X- and gamma-spectrometry and by accelerator mass spectrometry (AMS). In this work, we deal exemplarily with the production of 10Be, 22Na and 26Al from target elements (25≤Z≤28). These data are essential for an understanding of the production of cosmogenic nuclides by the interaction of galactic cosmic ray protons with matter. The new cross sections are compared with literature data and discussed in the context of Monte Carlo calculations describing the production cross sections by an intranuclear cascade evaporation model.