P. Signer

A model for the production of cosmogenic nuclides in chondrites

Abstract A model is presented which permits the calculation of production rates of cosmic-ray-produced light noble gases He, Ne and Ar as well as 10Be, 26Al and 53Mn in chondrites of variable size and shape The production of a given nuclide is described by a semi-empirical equation used by Signer and Nier (1960) to describe the production of the light noble gases in the iron meteorite Grant. For a given nuclide, this equation contains two free parameters that are fitted to the depth profiles measured in the Knyahinya chondrite. Model predictions are tested by comparison with literature data for depth profiles in other meteorites (ALH78084, St. Severin, Keyes). The agreement between model predictions and experimental data is found to be 5% for the concentrations of 10Be, 21Ne, 22Ne, 38Ar and 53Mn, respectively and to be better than 1% for the 22 Ne 21 Ne ratios. The model predicts that 3-nuclide correlations that use the same reference nuclide are linear. The correlation between P( 10 Be) P( 21 Ne) and 22 Ne 21 Ne ratios is experimentally verified over a wide range of irradiation conditions. With this relation, shielding and size corrected exposure ages can be derived from the measurements of 10Be and of the Ne isotopes in a single sample. The calculated exposure ages are believed to have a precision of 5%.

Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals

He, Ne, and Ar abundances and isotopic ratios in plagioclase and pyroxene separates from lunar soils were determined using a closed system stepwise etching technique. This method of noble gas release allows one to separate solar wind (SW) noble gases from those implanted as solar energetic particles (SEP). SEP-Ne with 20Ne22Ne = 11.3 ± 0.3 is present in all samples studied. The abundances of SEP-Ne are 2–4 orders of magnitude too high to be explained exclusively as implanted solar flare gas. The major part of SEP-Ne possibly originates from solar “suprathermal ions” with energies < 0.1 MeV/amu. The isotopic composition of Ne in these lower energy SEP is, however, probably identical to that of real flare Ne. The suggestion that SEP-Ne might have the same isotopic composition as planetary Ne and thus possibly represent an unfractionated sample of solar Ne is not tenable. SW-Ne retained in plagioclase and pyroxene is less fractionated than has been deduced by total fusion analyses. Ne-B is a mixture of SW-Ne and SEP-Ne rather than fractionated SW-Ne. In contrast to SEP-Ne, SEP-Ar has probably a very similar composition as SW-Ar.

Characterisation of Q-gases and other noble gas components in the Murchison meteorite

Abstract Noble gases in several HF/HCl resistant residues of the CM2 chondrite Murchison were measured by closed-system stepped etching, in order to study the planetary gases in their major carrier “Q”—an ill-defined minor phase, perhaps merely a set of adsorption sites. Neon, Ar, Kr, Xe, and probably also He in “Q” of Murchison have the same isotopic and nearly the same elemental abundances as their counterparts in Allende (CV3). The isotopic composition of Ne-Q is consistent with mass-dependent fractionation of either solar wind Ne or Ne from solar energetic particles. Unlike Allende, Murchison during HNO 3 attack releases, besides Q-gases, large amounts of two other Ne-components, Ne-E and Ne-A3, a third subcomponent of Ne-A. This work confirms that Q-gases of well-defined composition were an important noble gas component in the early solar system and are now found in various classes of meteorites, such as carbonaceous chondrites, ureilites, and ordinary chondrites. Ne-Q may have played a role in the formation of noble gas reservoirs in terrestrial planets.

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.

Exposure history of the regolithic chondrite Fayetteville: I. Solar-gas-rich matrix☆

Solar gas bearing matrix samples of the H4 chondrite Fayetteville contain up to 45% more cosmogenic 21Ne than some of the solar-gas-free light inclusions. Concentrations of solar and cosmogenic Ne in the matrix samples correlate. The excesses of cosmogenic Ne are comparable to those found in two out of three groups of track-rich grains of Fayetteville studied by Caffee et al. (1986). The excesses can be explained by a GCR-irradiation of a considerable part, if not all, of the matrix grains in their parent body regolith during at least some 20 Ma. We see no need to postulate a T-Tauri type solar cosmic ray irradiation of Fayettevilles regolith. The parent body exposures derived here are considerably longer than the < 1 Ma predicted by the model on the evolution of asteroidal regoliths by housen et al. (1979). The ratio of solar to cosmogenic gas acquired in the regolith points to an orbit of Fayettevilles parent body at some 2–3 AU.

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.

Production of stable and radioactive nuclides in thick stony targets (R = 15 and 25 cm) isotropically irradiated with 600 MeV protons and simulation of the production of cosmogenic nuclides in meteorites

Abstract Two artificial meteoroids made out of gabbro with a density of 3 g cm−3 with radii of 15 and 25 cm were isotropically irradiated with 600 MeV protons in order to simulate the production in meteoroids of cosmogonie nuclides by galactic cosmic ray protons. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, Lu, and Au was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si as well as from degassed meteoritic material was determined. Together with earlier results on an artificial meteoroid with a radius of 5 cm, and with data derived from thin-target experiments, the depth dependence of production rates is investigated for radii from 0 to 75 g cm−2. 60Co from Co shows the strongest size dependence; the center production rates differ by a factor of 100 for radii of 5 and 25 cm. Other low-energy products, like 58Co from Co and 24Na produced from Al, increase only up to a factor of 3.5 over the entire range of radii. For extreme high-energy products, in contrast, the center production rates decrease by up to a factor of 10. The observed depth profiles show a wide varity of shapes. Low-energy products have pronounced maxima in the center, high-energy products exhibit strong decreases from surface to center and, in between, essentially flat profiles are seen as well as such with a transition maximum. The spectra of primary protons and of secondary protons and neutrons in the artificial meteoroids were calculated using Monte Carlo techniques. The fluxes of secondary protons and neutrons depend strongly on depth and size, the spectral shapes being different for protons and neutrons. Calculating also the nucleon spectra which result from irradiation with real GCR p-spectra, the differences between simulation experiments and cosmic irradiation conditions are quantitatively described. On the basis of all these spectra and of thin-target excitation functions, production rates were calculated and compared with the experimental ones. The theoretical depth profiles allow to distinguish the different contributions of primary and secondary particles and to unravel the various production modes of cosmogenic nuclides in meteoroids. Our investigation shows that it is possible to model the production of residual nuclides in artificial meteoroids with excellent accuracy by thin-target calculations, provided that reliable thin-target excitation functions are at hand.

On the depth dependence of spallation reactions in a spherical thick diorite target homogeneously irradiated by 600 MeV protons: Simulation of production of cosmogenic nuclides in small meteorites

Abstract An artificial meteorite made out of diorite with a radius of 5 cm was irradiated isotropically with 600 MeV protons in order to simulate the production of cosmogenic nuclides in small meteorites by galactic cosmic ray protons. The primary proton dose of 4.82 × 1015 cm−2 is equivalent to a some 50 Ma of cosmic ray exposure. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, and Lu was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si, as well as from glass and meteoritic material was determined. Thick-target data are compared with thin-target production rates measured simultaneously, thus allowing to separate the contribution of secondary protons and neutrons. The results demonstrate that, in contrast to present presumptions, even in small meteorites secondary particles have to be considered and that the depth profiles show differences in production of up to 30% between surface and center. The experimental data are discussed with respect to cosmic irradiation conditions of real meteorites. Using Monte Carlo techniques depth dependent nucleon spectra were calculated. On the basis of these spectra and of thin-target excitation functions theoretical production rates were derived and compared with the experimental ones. This comparison shows that it is possible to reproduce the experimental depth profiles quite well by thin-target calculations provided reliable excitation functions are at hand. The thick-target measurements and the thin-target calculations provide a basis for an advanced modelling of the production of cosmogenic nuclides in small meteorites, which strictly differentiates between all contributing production modes.

Uranium-xenon chronology: precise determination of λsƒ ∗136Ysƒ for spontaneous fission of 238U

UXe chronology requires knowledge of the product of the decay constant (λsƒ) and the fractional yield of 136Xe (136Ysƒ) produced by the spontaneous fission of 238U. However, values derived from U oxides are in conflict with those obtained from U-bearing accessory minerals. We present combined UPb and UXe data for a carefully selected suite of zircon and monazite separates. All samples have precise, concordant or near-concordant UPb ages and yield a λsƒ ∗136Ysƒ value of (6.83 ± 0.18) × 10−18 a−1. Recoil loss of fissiogenic Xe was corrected for numerically or circumvented by abrading samples or analysing very large crystals. Old crystal cores that might have retained their fission Xe but not their radiogenic Pb were not detected in any of the samples. No significant contribution of Xe from 232Th fission is expected as long as the ThU ratio is ⩽ 10. Taken together, these observations indicate that the derived value for λsƒ ∗136Ysƒ is correct. This value is similar to that from earlier studies of accessory minerals, but some 20% higher than that determined recently from carefully selected U minerals, including pitchblende, uraninite and coffinite. This suggests that U oxides systematically lose fissiogenic Xe, possibly due to regeneration of the crystal lattice or changes in oxidation state. UXe ages from such minerals should be interpreted with caution.

Trapping of the solar wind in solids: Part I. Trapping probability of low energy He, Ne and Ar ions

Abstract The trapping probability in Al foils of He, Ne and Ar ions with energies between 0.46 and 7 keV has been determined. Argon is trapped quantitatively for ion energies higher than 2.5 keV. At 2.5 keV the trapping probabilities of neon and helium are about 0.6. For lower energies the trapping probability of argon decreases rapidly. No saturation effects were observed for integrated bombardment fluxes up to 2 × 10 14 Ar ions/cm 2 at 2.5 keV and 3 × 10 13 Ar ions/cm 2 at 0.46 keV. The trapped gases are held quite firmly. Foils bombarded with ions of 2.5 and 7.5 keV were heated at 200 and 400°C for times up to 650 hours. Only 10–20% of the helium and virtually no neon and argon were lost at 200°C. However, at 400°C, gas losses become quite serious within 24 hours. These results show that an Al foil exposed to the solar wind would trap and retain a large fraction of the solar wind ions.