Ingo Leya

Cosmogenic noble gas studies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctica

Extraordinarily high surface exposure ages have been determined for Sirius Group tillites of Mt. Fleming and Mt. Feather as well as at localities in the Inner Dry Valleys using cosmogenic helium and neon. Ages of 10 Ma at Mt. Fleming, 5.3 Ma at Mt. Feather and 6.5 Ma at Insel Mountain are among the highest nominal exposure ages published so far. These values are minimal ages as they are based on the assumption of zero erosion and uplift. The Mt. Feather sample independently confirms the pre-Pliocene age of the Sirius Group sediments in the Dry Valleys as previously determined at Mt. Fleming. The Insel Mountain samples provide evidence for a landscape formation of the Inner Dry Valleys not later than Late Miocene time. Assuming conservatively low values of 2.5 cm Ma−1 for erosion rate and 50 m Ma−1 for uplift rate we infer that the Sirius Group tillites at Mt. Fleming were deposited earlier than 20 Ma ago. This indicates that the overriding of the Dry Valleys block of the Transantarctic Mountains by the East Antarctic Ice Sheet occurred not later than the Early Miocene. Maximum long-term erosion rates in the Inner Dry Valleys must be <15 cm Ma−1 down to altitudes <1000 m. Since such low erosion rates require permanently cold and hyperarid conditions, the response of Antarctica to the Pliocene warm climatic episode must have been small. Cosmogenic nuclide data from both the Inner Dry Valleys and the Sirius Group sediment localities support the hypothesis of a stable East Antarctic Ice Sheet since at least Late Miocene time, implying that the climate of Antarctica was decoupled from that of lower southern latitudes. We present also new elemental 21Ne production rates of P21(Mg) = 196 atoms g−1 yr−1 and P21(Al) = 55 atoms g−1 yr−1 at sea level and high geomagnetic latitude. These figures are consistent with a 3He production rate of P3 = 110 atoms g−1 yr−1, similar to previously published values. This consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 Ma. Cosmogenic Ne in quartz and pyroxene has a (22Ne/21Ne)cos ratio of 1.266 ± 0.040 and 1.159 ± 0.040, respectively.

Cosmic-ray production of tungsten isotopes in lunar samples and meteorites and its implications for Hf–W cosmochemistry

Excesses and deficiencies in 182W in meteorites and lunar samples relative to the terrestrial 182W atomic abundance have been assigned to the decay of 182Hf (t1/2=9 Ma) and have been used to date metal-silicate fractionation events in the early solar system. Because the effects are very small, production and burn-out of tungsten isotopes by cosmic ray interactions are a concern in such studies. Masarik [J. Masarik, Contribution of neutron-capture reactions to observed tungsten isotopic ratios, Earth Planet. Sci. Lett. 152 (1997) 181–185] showed that neutron-capture reactions on tungsten isotopes can account at best for a minor part of the observed deficit of 182W in Toluca and other iron meteorites. On the other hand, in lunar samples and stony meteorites the production of 182W from 181Ta may become crucial. Here, we calculate this contribution as well as production and consumption of 182–186W by other neutron-induced reactions. The neutron fluence of each sample is estimated by its nominal cosmic-ray exposure age deduced from noble gas data. This approach overestimates the true cosmogenic W isotopic shifts for samples that might have been irradiated very close to the regolith surface. A quantitative estimate is often also hampered by a lack of Ta data. Despite these reservations, it appears that in many lunar samples neutron-capture on Ta has caused a large part of the observed 182W excess. On the other hand, in some samples, especially those with very low exposure ages, clearly only a minor or even negligible fraction of the 182W excess can be cosmogenic. Therefore, the conclusion, based on Hf–W model ages, that the Moon formed 50 Myr after the start of the solar system remains valid. Martian meteorites have lower Ta/W ratios and cosmic ray exposure ages than most lunar samples. Therefore, cosmogenic production has not significantly altered the W isotopic composition in Martian meteorites. Observed 182W excesses in Martian meteorites as well as the very large excesses in two eucrites are undoubtedly the result of early 182Hf decay.

THE PREDICTABLE COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS BY SPALLATION REACTIONS IN THE EARLY SOLAR SYSTEM

Ever since their first discovery in 1960, the origin of the relatively short-lived radionuclides, now extinct but alive in the early solar system, has been under debate. Possible scenarios are either nucleosynthetic production in stellar sources, e.g., asymptotic giant branch stars, Wolf-Rayet stars, novae, and supernovae, with subsequent injection into the solar nebula, or the production by spallation reactions in the early solar system. Here we present model calculations for the second scenario, the production of the relatively short-lived radionuclides by solar energetic particle events at the start of the solar system. The model is based on our current best knowledge of the nuclear reaction probabilities. In addition, the modeling depends on the relative fluence contribution of protons, 3He, and 4He in the solar particle events as well as on their energy distribution. The relative fluence contribution is the only free parameter in the system. Finally, the modeling depends on the chemical composition assumed for the irradiated target. The model simultaneously describes the observed solar system initial ratios 7Be/9Be, 10Be/9Be, 26Al/27Al, 41Ca/40Ca, 53Mn/55Mn, and 92Nb/93Nb. In the framework of the local production scenario, the concordance of measured and modeled data for nuclides with half-lives ranging from 53 days up to 36 Myr enables us to put some stringent constraints on possible calcium-aluminum-rich refractory inclusion (CAI) production and its timing. One important requirement in such a scenario is that the material forming most of the CAIs must have experienced a surprisingly homogenous particle fluence. CAIs showing evidence for live 10Be, 26Al, 41Ca, 53Mn, and 92Nb close to the inferred solar system initial ratios would have to have been irradiated within ~1 Myr. Much more stringent would be the time constraint for the one CAI for which formerly live 7Be has been reported. Such CAIs would have to have been irradiated for less than about 1 yr. Such a short timescale requires flux densities as high as ~1016 cm-2 s-1. To allow further tests of the local production scenario, we also predict solar system initial ratios for 14C/12C, 22Na/23Na, 36Cl/35Cl, 44Ti/48Ti, 54Mn/55Mn, 63Ni/60Ni, and 91Nb/93Nb, whose correlated shifts in the daughter isotopes would help to further test the local production scenario.

The influence of cosmic-ray production on extinct nuclide systems

Variations in the atomic abundances of 53Cr, 92Zr, 98Ru, 99Ru, and 182W in meteorites and lunar samples relative to terrestrial values may imply the early decay of radioactive 53Mn, 92Nb, 98Tc, 99Tc and 182Hf, respectively. From this one can deduce nucleosynthetic sites and early solar system timescales. Because these effects are very small, production and consumption of the respective isotopes by cosmic-ray interactions is a concern. It has recently been demonstrated that 182W production by neutron capture reactions on 181Ta is crucial for most lunar samples (Leya et al., 2000a). In this study the neutron fluence of each sample was estimated from its nominal cosmic-ray exposure age as deduced from noble gas data. This approach overestimates the true cosmogenic isotopic shift for samples that might have been irradiated very close to the regolith surface. Here we therefore combine our model calculations with the neutron dose proxies 157Gd/158Gd and 149Sm/150Sm. This allows us to accurately correct the measured W isotopic data for cosmic-ray induced shifts without the explicit knowledge of the exposure age or the shielding depth of the sample simply by measuring 157Gd/158Gd and/or 149Sm/150Sm in an aliquot. In addition we present new model results for the GCR-induced effects on 53Mn-53Cr, 92Nb-92Zr and 98Tc-99Tc-98Ru-99Ru. For each of these systems, except Tc-Ru, a proper cosmic-ray dose proxy is given, permitting the accurate correction of measured isotopic ratios for cosmogenic contributions.

Cosmogenic tungsten and the origin and earliest differentiation of the Moon

The decay of formerly live 182Hf with a half-life of 9 Myr results in variations in the abundance of 182W in early solar system objects. Here we demonstrate that major excesses in 182W in some lunar samples are the results of cosmogenic additions. Apollo 17 high-Ti mare basalts yield high 182W/184W of up to ϵw=+11±1. Even more extreme variations of up to ϵw=+22±1 are found for mineral separates, although these lavas were erupted more than 500 Myr after the start of the solar system. The measured 182W excess in the separated minerals is correlated with their Ta/W, confirming theoretical models that implicate the 181Ta(n,γ)182Ta(β−)182W reaction from cosmic irradiation as the most likely cause. In contrast, olivine–basalt 15555, which has a low cosmic ray exposure age, displays no internal 182W variations and defines an ϵw of +1.3±0.4. This is consistent with earlier conclusions that the Moon formed about 50 Myr after the start of the solar system. The high-Ti mare basalt source, with very high Hf/W, has a W isotopic composition that is not grossly different, from which a time limit of ∼70 Myr after the start of the solar system can be inferred for the formation of ilmenite-rich layers in the final stages of the lunar magma ocean.

From stellar nebula to planets: The refractory components

Context. To date, calculations of planet formation have mainly focused on dynamics, and only a few have considered the chemical composition of refractory elements and compounds in the planetary bodies. While many studies have been concentrating on the chemical composition of volatile compounds (such as H2O, CO, CO2) incorporated in planets, only a few have considered the refractory materials as well, although they are of great importance for the formation of rocky planets. Aims. We computed the abundance of refractory elements in planetary bodies formed in stellar systems with a solar chemical composition by combining models of chemical composition and planet formation. We also considered the formation of refractory organic compounds, which have been ignored in previous studies on this topic. Methods. We used the commercial software package HSC Chemistry to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e., organic compounds are formed in the gas phase and are non-reactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase. Results. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides are present in the planets, and pyroxene and olivine are formed in similar quantities. The model predicts planets that are similar in composition to those of the solar system. Starting from a common initial nebula composition, it also shows that a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process. Conclusions. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.

Nucleogenic production of Ne isotopes in Earth's crust and upper mantle induced by alpha particles from the decay of U and Th

The production of nucleogenic Ne in terrestrial crust and upper mantle by alpha particles from the decay of U and Th was calculated. The calculations are based on stopping powers for the chemical compounds and thin-target cross sections. This approach is more rigorous than earlier studies using thick-target yields for pure elements, since our results are independent of limiting assumptions about stopping-power ratios. Alpha induced reactions account for >99% of the Ne production in the crust and for most of the 20,21Ne in the upper mantle. On the other hand, our 22Ne value for the upper mantle is a lower limit because the reaction 25Mg(n,α)22Ne is significant in mantle material. Production rates calculated here for hypothetical crustal and upper mantle material with average major element composition and homogeneously distributed F, U, and Th are up to 100 times higher than data presented by Kyser and Rison [1982] but agree within error limits with the results by Yatsevich and Honda [1997]. Production of nucleogenic Ne in “mean” crust and mantle is also given as a function of the weight fractions of O and F. The alpha dose is calculated by radiogenic 4He as well as by the more retentive fissiogenic 136Xe. U and Th is concentrated in certain accessory minerals. Since the ranges of alpha particles from the three decay chains are comparable to mineral dimensions, most nucleogenic Ne is produced in U- and Th-rich minerals. Therefore nucleogenic Ne production in such accessories was also calculated. The calculated correlation between nucleogenic 21Ne and radiogenic 4He agrees well with experimental data for Earths crust and accessories. Also, the calculated 22Ne/4He ratios as function of the F concentration and the dependence of 21Ne/22Ne from O/F for zircon and apatite agree with measurements.

Cross sections for the proton-induced production of He and Ne isotopes from magnesium, aluminum, and silicon

Abstract We measured integral thin target cross sections for the proton-induced production of the rare gas isotopes 3 He, 4 He, 20 Ne, 21 Ne and 22 Ne from Mg, Al, and Si from the respective reaction threshold up to 1.6 GeV. These target elements were chosen since they account for more than 50% and 95% of the cosmogenic He and Ne production in extraterrestrial matter, respectively. In order to minimize the influences of secondary particles on the production of residual nuclides a so-called “mini-stack”-approach was used instead of the well known “stacked-foil-technique” for all irradiation experiments with proton energies above 200 MeV. With this new data base a complete and consistent set of excitation functions for the proton-induced production of He and Ne isotopes is established for all target elements relevant for deciphering the cosmic ray record in extraterrestrial matter.

NEW TITANIUM ISOTOPE DATA FOR ALLENDE AND EFREMOVKA CAIs

We measured the titanium (Ti) isotope composition, i.e., 50Ti/47Ti, 48Ti/47Ti, and 46Ti/47Ti, in five calcium-rich-aluminum-rich refractory inclusions (CAIs) from the oxidized CV3 chondrite Allende and in two CAIs from the reduced CV3 chondrite Efremovka. Our data indicate that CAIs are enriched in 50Ti/47Ti and 46Ti/47Ti and are slightly depleted in 48Ti/47Ti compared to normal Ti defined by ordinary chondrites, eucrites, ureilites, mesosiderites, Earth, Moon, and Mars. Some CAIs have an additional 50Ti excess of ~8e relative to bulk carbonaceous chondrites, which are enriched in 50Ti by ~2e relative to terrestrial values, leading to a total excess of ~10e. This additional 50Ti excess is correlated with nucleosynthetic anomalies found in 62Ni and 96Zr, all indicating an origin from a neutron-rich stellar source. Bulk carbonaceous chondrites show a similar trend, however, the extent of the anomalies is either less than or similar to the smallest anomalies seen in CAIs. Mass balance calculations suggest that bulk Allende Ti possibly consists of a mixture of at least two Ti components, anomalous Ti located in CAIs and a normal component possibly for matrix and chondrules. This argues for a heterogeneous distribution of Ti isotopes in the solar system. The finding that anomalous Ti is concentrated in CAIs suggests that CAIs formed in a specific region of the solar system and were, after their formation, not homogeneously redistributed within the solar system. Combining the CAI data with improved model predictions for early solar system irradiation effects indicates that a local production scenario for the relatively short lived radionuclides can be excluded, because the production of, e.g., 10Be, 26Al, and 41Ca, would result in a significant collateral shift in Ti isotopes, which is not seen in the measured data.

Elemental ratios in stars vs planets

Context. The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same. Aims. Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O. Methods. A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances. Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.