26Al (dpm/kg) and or He, Ne, and Ar have been measured in samples of the Norton County meteorite from three different depths. The gas contents yield a shielding-corrected age of 75.8±5.7 m.y. The ratio of atomic production rates P(26Al )/P(21Ne) is 0.22±0.02. 3He, 21Ne, and 26Al increase with decreasing 22Ne/21Ne at nearly the same rates as they do in the smaller Keyes chondrite. 26Al contents were measured in Bustee (75.9±3.3), Khor Temiki (89.9±3.5), Pesyanoe (98.4±3.4), and Shallowater (62.8±2.0) meteorites. 26Al/26Alo correlates with 22Ne/21Ne, and comparison with the corresponding chondrite correlation indicates that the aubrites, like Malakal, may have experienced an unusually high flux of cosmic rays.
The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.
Measurements of the 10 Be and 26 Al contents of ALHA 81005 constrain the length and conditions of its exposure to cosmic rays. Calculations based on one‐step irradiation models imply that the time spent by this object in space is shorter than that spent by most ‘asteroidal’ meteorites. On the other hand the results are readily consistent with a lunar origin for ALHA 81005.
Abstract— We report measurements of 26 Al and 10 Be activities in nine ordinary chondrites and of the light noble gas concentrations and 36 Cl and 41 Ca activities in subsets of those meteorites. All but Murray have low 21 Ne concentrations (<1.0 × 10 −8 cm 3 STP/g) and have previously been used to estimate 21 Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21 Ne production rates that are consistent with the standard L‐chondrite value of 0.33 × 10 −8 cm 3 STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower 3 He/ 21 Ne ratios than expected based on their 22 Ne/ 21 Ne ratios. A general association between low 21 Ne contents and 3 He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low 21 Ne contents, one‐stage exposure histories, and losses of cosmogenic 3 He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia.
We have measured significant concentrations of 36 Cl, 41 Ca, 36 Ar from decay of 36 Cl, and 150 Sm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 36 Cl and 41 Ca, corrected for a high‐energy spallogenic component and a terrestrial age of ∼50 ka, give average neutron‐capture production rates of 208 atoms/min/g‐Cl and 1525 atoms/min/kg‐Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm 2 /s and 4.3 n/cm 2 /s, respectively. If sustained for the ∼65 Ma single‐stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ∼1.3×10 16 and 0.8 × 10 16 n/cm 2 for 36 Cl and 41 Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 36 Ar/ 38 Ar ratios as large as ∼9. The correlation of high 36 Ar/ 38 Ar with high Cl/Ca phases in neutron‐irradiated Chico indicates that the excess 36 Ar above that expected from spallation is due to decay of neutron‐produced 36 Cl. Excess 36 Ar in Chico requires a thermal neutron fluence of 0.9–1.7×10 16 n/cm 2 . Decreases in 149 Sm/ 152 Sm due to neutron‐capture by 149 Sm correlate with increases in 150 Sm/ 152 Sm for three samples of Chico, and one of the Torino H‐chondrite. The 0.08% decrease in 149 Sm/ 152 Sm shown by Chico corresponds to a neutron fluence of 1.23×10 16 n/cm 2 . This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 36 Ar identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of ∼0.2–0.6×10 16 n/cm 2 . Depletion of 149 Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8×10 16 n/cm 2 and 0.25×10 16 n/cm 2 , respectively. Neutron fluences of ∼10 16 n/cm 2 in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron‐capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron‐capture has the potential for identifying large meteorites with complex exposure histories.
The nuclides made in extraterrestrial materials by cosmic rays help reveal the histories both of the irradiated objects and of the cosmic rays. Improvements in measurement techniques, especially in accelerator mass spectrometry, have greatly reduced detection limits. Thanks to several extensive series of measurements in meteorites, a comprehensive picture has taken shape of how cosmogenic nuclide production depends on size, shape, and composition. Complementary to this work, (1) the irradiation in space of meteoroids has been simulated by means of accelerator experiments both with very thick and with spherical targets, and (2) various models for calculating production rates of cosmogenic nuclides have been developed or refined. New classes of material—meteorites recovered in Antarctica and tiny meteorites from the stratosphere and deep‐sea sediments, for example—have become more widely available for analysis. From their cosmogenic nuclide contents we have learned about the exposure histories of lunar meteorites, of SNC meteorites, which may come from Mars, and of interplanetary dust particles. Cosmogenic nuclides have been put to use in unfolding increasingly complex exposure histories. Some grains in gas‐rich meteorites were irradiated in at least two episodes, and others perhaps even before the solar system formed. Continuing measurements of lunar samples reveal the past behavior of energetic solar particles. We now know from cosmogenic nuclide measurements that many meteorites found in Antarctica have been there for ∼0.1–1 million years and that the age distributions vary from site to site.
Abstract— The original mass (15915 g) of the Twannberg IIG (low Ni‐, high P) iron was found in 1984. Five additional masses (12 to 2488 g) were recovered between 2000 and 2007 in the area. The different masses show identical mineralogy consisting of kamacite single crystals with inclusions of three types of schreibersite crystals: cm‐sized skeletal (10.5% Ni), lamellar (17.2% Ni), and 1–3 × 10 μm‐sized microprismatic (23.9% Ni). Masses I and II were compared in detail and have virtually identical microstructure, hardness, chemical composition, cosmic‐ray exposure (CRE) ages, and 10 Be and 26 Al activities. Bulk concentrations of 5.2% Ni and 2.0% P were calculated. The preatmospheric mass is estimated to have been at least 11,000 kg. The average CRE age for the different Twannberg samples is 230 ± 50 Ma. Detrital terrestrial mineral grains in the oxide rinds of the three larger masses indicate that they oxidized while they were incorporated in a glacial till deposited by the Rhône glacier during the last glaciation (Würm). The find location of mass I is located at the limit of glaciation where the meteorite may have deposited after transport by the glacier over considerable distance. All evidence indicates pairing of the six masses, which may be part of a larger shower as is indicated by the large inferred pre‐atmospheric mass.
