Sherri K. Vogt

Cosmogenic nuclides and nuclear tracks in the chondrite Knyahinya

Abstract Cosmic-ray produced He, Ne, Ar as well as 10Be, 26Al, 53Mn and nuclear tracks were determined in samples from known positions within the L5 chondrite Knyahinya. Our results show that Knyahinya experienced a single-stage exposure history as a meteoroid of approximately spherical shape. The inferred preatmospheric mass was about 1300–1400 kg, corresponding to a mean radius of 45 cm. Good agreement is observed between the preatmospheric shapes derived from cosmic-ray track densities and from 22 Ne 21 Ne ratios. The exposure age of Knyahinya is 40.5 Ma. The 10 Be 21 Ne ratios are constant, although the concentrations of both nuclides increase by more than 20% from positions of lowest to highest shielding. The constancy of this ratio allows refined shielding corrections in computations of exposure ages. High 26Al activities of up to 77 dpm/kg indicate that this nuclide is more efficiently produced by low energy particles than is 21Ne.

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.

Exposure history of the regolithic chondrite Fayetteville: II. Solar-gas-free light inclusions

Abstract Noble gases and cosmic-ray tracks were determined on light, solar-gas-free inclusions of the solar-gas-rich H4 chondrite Fayetteville. 10 Be, 26 A1, major and trace element concentrations as well as oxygen isotope ratios complete the data set. One exceptional inclusion is an L-chondrite xenolith, identified by its oxygen isotope signature and the concentration of metallic Fe-Ni. Oxidized Fe of this inclusion equilibrated with the host early in the history of Fayettevilles parent body. Track density variations indicate that the majority of Fayettevilles inclusions were irradiated as pebbles in the parent body by solar flare particles for about 10 4 -10 6 years. The L-chondritic inclusion contains an unequivocal excess of GCR produced He and Ne relative to the remaining inclusions. This clast, as well as two more solar-gas-free samples suffered a GCR exposure on the parent body lasting some 10 million years or longer. This exposure is comparable in length to the residence times of solar-gas-rich matrix samples in the GCR active zone of the Fayetteville parent regolith. Probably the other inclusions also were irradiated by GCR during at least several million years prior to the separation of the meteoroid from the parent body.

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.

Determination of spallogenic long-lived radionuclides in chondrites by nuclear analytical techniques

The record of the interaction of cosmic ray particles with meteorites in space is preserved in these objects in the form of radioactive and stable nuclides. The long-lived radionuclides 10Be (t½= 1.6 × 106 years), 26Al (t½= 7.2 × 105 years) and 53Mn (t½= 3.8 × 106 years) were determined in a large number of non-Antarctic and Antarctic chondrites by means of accelerator mass spectrometry, gamma-gamma coincidence techniques and neutron activation analysis, respectively. Radiochemical separations with high decontamination factors of interfering isotopes were used. Detection limits down to 1 × 10–14 g g–1 were achieved for isotope ratio measurements. For absolute analytical techniques the detection limit could be lowered to 10–14 g g–1. The radionuclide data obtained revealed the relatively low exposure ages of the non-Antarctic H and L chondrites investigated. With the exception of the Jilin H5 chondrite the results also indicated the simple irradiation histories of these meteorites in space. The terrestrial residence times and possible genetic relationships of the Antarctic chondrites were determined; this is of interest for the elucidation of the common origin of these objects.