K. Marti

The 36Cl–36Ar–40K–41K records and cosmic ray production rates in iron meteorites

This is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the last billion years, as recorded in these fossil detectors. We have performed new 36Cl, 26Al, 10Be, and noble gas measurements, including determination of the shielding parameter, S = 4He/21Ne, in samples with published K data. The K isotopic data, coupled to 36Ar and 36Cl concentrations permit selection of meteorites which have only experienced simple (constant geometry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. In order to carry out production rate calibrations based on 40K–41K data, we assume constancy of the cosmic flux during the interval 150 to 700 My ago. We note that meteorites with very old potassium ages cannot be included in this calibration, as these meteorites require distinct parameter sets. A calibration data set representing a total of 13 meteorites was used to compute long-term (0.5 Gy) average production rates. These average production rates of 36Cl from this particular calibration set are significantly (28%) lower than those determined for the recent (≤10 My) cosmic ray flux. We also document here the quality of the resulting potassium production rate parameter M0(S) with a calculated isochron for irons of group IVA.

Mass spectrometric identification of cosmic-ray-produced neon in terrestrial rocks with multiple neon components

Studies of cosmic-ray-produced nuclides in terrestrial rocks are expected to provide important geomorphological and glaciological information, such as surface exposure ages, erosion rates and extent of glacial cover. In the case of the stable nuclide21Ne, the cosmic-ray-produced component may represent but one of several components, and a component resolution based on three-isotope correlations is essential. We developed analytical techniques which permit accurate corrections for all interfering ions. We show (for our modified Nier source) that corrections for doubly charged species based on constant charge state ratios are not satisfactory. The improved techniques permit isotopic studies of < 106 atoms of21Ne. The spallation ratio(22Ne/21Ne)c from Si was calibrated using a quartz separate from an Allan Hills sandstone and yields a value of1.243 ± 0.022. This ratio is expected to be essentially constant for terrestrial quartz samples and should be useful in component resolutions. The stepwise release of Ne at increasing temperatures shows that cosmic-ray-produced Ne is released from quartz at rather low temperatures. This permits an estimate of the activation energy for Ne diffusion in quartz (90 ± 10 kJ/mol).

Cosmic-ray-produced 21Ne in terrestrial quartz: the neon inventory of Sierra Nevada quartz separates

The study of cosmic-ray-produced radioactive and stable nuclides on the surface of the Earth can provide relevant geomorphological and glaciological information. At present, the cosmic ray production rates of stable 21Ne are not well known. This study attempts to remedy the situation by determining the production rate ratio of 21Ne and 26Al, P21P26, in quartz. 26Al concentrations and P26 rates have previously been investigated for quartz separates of Sierra Nevada rocks which were brought to the surface by glacial scouring during the Tioga period at the end of the last ice age [1]. We used splits of the same samples for our studies and found that Ne in these rocks represents a mixture of several components: trapped Ne, nucleogenic 21Ne and 22Ne produced by (α,n) reactions in oxygen and fluorine, respectively, as well as cosmic-ray-produced Ne, which is the component of interest in this study. The trapped component was substantially lost in one sample (W86-12) by crushing and by a density separation of the grain sizes 38–90 μm and 90–125 μm, permitting the resolution of the in situ produced 21Ne into cosmic-ray spallation and (α,n) produced components and the determination of a lower limit to P21P26. In a second sample (W86-8) one split contained small enough amounts of nucleogenic 21Ne to permit the determination of a reasonable upper limit to P21P26. The two ratio determinations are consistent within error limits and the value adopted, 0.65 ± 0.11 (2σ), agrees with ratios observed in extraterrestrial matter. Apparently, P21P26 is thus not very sensitive to the neutron spectrum. However, the observed production rate ratio is substantially larger than theoretical estimates for Si targets, reflecting poorly known neutron excitation functions. The above P21P26 value, coupled to the observed 26Al production rate [1], corresponds to a 21Ne production rate of P21 = 21 atoms g−1 a−1 in quartz or to P21 = 45 atoms (g Si)−1 a−1 (at sea level and high latitudes). This rate is based on an adopted exposure age of 11,000 yr for our quartz samples.

Volatiles (nitrogen, noble gases) in recently discovered SNC meteorites, extinct radioactivities and evolution ☆

Abstract We report noble gas and nitrogen analyses of newly discovered SNC meteorites, one nakhlite (NWA817) and four shergottites (NWA480, NWA856, NWA1068, and SaU 005). The K–Ar age (1.3 Ga) as well as the cosmic-ray exposure (CRE) age (10.0±1.3 Ma) of nakhlite NWA817 agree with data of Nakhla. The CRE ages of NWA480, NWA856, and NWA1068 (2.35±0.20, 2.60±0.21 and 2.01±0.65 Ma, respectively) are consistent, within uncertainties, with other basaltic shergottites, but the CRE age of SaU 005 (1.25±0.07 Ma) is distinct and indicates a different ejection event. Bulk K–Ar ages of all shergottites exceed the reported radiometric ages and reveal the presence of inherited radiogenic 40 Ar in basaltic lavas. The isotopic composition of nitrogen trapped in these SNC meteorites is not homogeneous, since δ 15 N values of either +15 to 20‰ or +45‰, indicate different nitrogen reservoirs. All shergottites contain fission xenon from 238 U, and fission Xe of extinct ( T 1/2 =82 Ma) 244 Pu, previously identified in ALH84001, in Chassigny and in Nakhla is also present in at least one shergottite (NWA856). The shergottites contain less fissiogenic Xe than other SNC, suggesting that either their source was more degassed or that the magma source region closed at a later time. In nakhlites, fission xenon from 244 Pu correlates with uranium, a geochemical proxy of plutonium. Thus it is possible that fissiogenic Xe was not inherited during magma differentiation, but rather was produced in situ and retained in refractory mineral assemblages. In this interpretation, the magma evolution that settled the mineralogy and geochemistry of nakhlites took place at a time when 244 Pu was alive and pre-dated the (late) events recorded in their radiometric ages. Alternatively, fissiogenic xenon was trapped from a mantle source during closed system evolution of the parent magmas, in which case such evolution might have taken place at considerable depth (pressure) in order to inhibit magma degassing during the course of differentiation.

Lunar nitrogen: indigenous signature and cosmic-ray production rate

Abstract Indigenous lunar nitrogen composition and abundances have been determined in old ferroan anorthosite 60025 and in anorthositic breccia 67915 from North Ray Crater, as well as in 55 cm deep volcanic glasses of 74001 double-drive core from Shorty Crater. Also included in the set is the well-documented lunar basalt 75075, collected near the Camelot Crater. Indigenous lunar N abundances are low (at ppm level), but there is some variation between glass-rich cores, mare basalts and anorthosites. The uniform indigenous N isotopic signature of δ 15 N=+13.0±1.2‰, is consistent with data reported previously for Shorty Crater samples. The indigenous δ 15 N cannot account for the light nitrogen component, observed in the lunar regolith samples. We have determined cosmic-ray production rates P ( 15 N) for the above rocks and the drill core samples. The average production rate estimate (for low shielding) of P ( 15 N)=5.8±0.6 pg 15 N/g/Ma is ∼60% higher than published lunar 15 N production rates, but consistent with the meteoritic production rate derived from silicates in the Enon meteorite, when normalized to 2π-irradiation geometry. From the observed cosmogenic 15 N excesses and the reported cosmogenic 21 Ne abundances in core 74001 we derive a ( 15 N C / 21 Ne C ) production rate ratio of 4.0±0.3 for silicates.

Martian atmospheric and interior volatiles in the meteorite Nakhla

Abstract We report isotopic abundances of xenon, argon, and nitrogen for the observed distinct components in Martian meteorite Nakhla. In a stepwise release the 129Xe/132Xe ratios in the 800°C and 1000°C fractions show the signature of the modern atmosphere and other Xe ratios, when corrected for the cosmogenic component, confirm the isotopically fractionated modern atmospheric component. On the other hand, the Xe isotope ratios in the >1000°C steps reveal an isotopically unfractionated interior component, but with radiogenic 129Xe excesses. The isotopic composition of this interior component is consistent with Chass-S Xe (solar type), but is augmented by 244Pu-derived fission Xe. The fission components in interior trapped Xe (in both Nakhla and Chassigny) suggest that Mars effectively retained 244Pu-derived fission gas. A heavy (relative to interior N) nitrogen signature in the 600–900°C temperature steps also suggests a recent incorporation of Martian atmospheric gases. The N signatures in the high-temperature (>1000°C) steps are strongly affected by cosmic-ray-produced 15Nc and 36Ar and 38Ar abundances are dominated by the cosmogenic component. We discuss the signatures of Martian interior nitrogen and heavy noble gases and the constraints they provide on the evolutionary history of Mars.