Abstract— We measured cosmic‐ray products—noble gases, radionuclides, thermoluminescence, and nuclear tracks—and trace element contents and mineralogy of samples of three orthogonal and mutually intersecting cores (41–46 cm long) of a 101.6 kg Ghubara individual (1958,805) at The Natural History Museum, London. The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked. While contents of noble gases and degree of shock‐loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two‐generation regolith breccia. Contents of cosmogenic 26Al and 10Be and low track densities indicate that the Ghubara individuals were located more than 15 cm below the surface of an 85 cm meteoroid. Because of its large size, Ghubara's cosmic‐ray exposure age is poorly defined to be 15–20 Ma from cosmogenic nuclides. Ghubara's terrestrial age, based on 14C data, is 2–3 ka. Not only is Ghubara the first known case of a two‐generation regolith breccia on the macroscale, it also has a complicated thermal and irradiation history.

We measured the long‐lived cosmogenic radionuclides 10Be, 26Al, and 36Cl in 47 H chondrite falls: 13 “Cluster 1” members, 9 “Cluster 5” members, and 25 random falls. From the date and time of fall, Clusters 1 and 5 were previously identified as possible coorbital meteoroid streams with distinctive thermal histories being confirmed by contents of volatile trace elements. Here, we use model data, including a three‐radionuclide plot (10Bebulk/26Albulk versus 36Clmetal/26Albulk) and the multivariate statistical techniques of logistic regression and linear discriminant analysis to compare radionuclide levels and their utility to differentiate specific suites from other H chondrites. From our radionuclide results and from noble gas data from other workers, we identified 35 falls with simple irradiation histories and cosmic ray exposure ages >4 Ma. Eight others exhibit evidence for shorter (≤4 Ma) exposure, three of which had complex exposure histories (two having been reported by others previously); three others may have had such a history. In any event, the small proportion of H chondrite falls with complex exposure histories supports recent suggestions that they are not commonly encountered, as earlier workers suggested. Although cosmogenic radionuclides do not differentiate between Cluster 1 and a random set of H chondrites, H chondrites that lost 3He from solar heating are distinguishable from those with normal 3He levels.

The compositionally typical H5 chondrite St-Robert has an exposure age, 7.8 Ma, indistinguishable from that of the main cluster of H chondrites. Small values of the cosmogenic Ne-22/Ne-21 ratio in interior samples imply a pre-atmospheric radius on the order of 40 cm. Sample depths based on tracks and the production rates of Bhattacharya et al. (1973) range from 6 to similar to 40 cm and are generally larger than depths estimated from published Co-60 activities, perhaps because the track production rates adopted are too high. Depth profiles of the production rates of C-14, Cl-36, Al-26, Be-10, and Ne-21 in stony material show increases with depth and reach levels 5% to 15% higher than expected from modeling calculations. The maximum concentrations in St-Robert are, however, generally comparable to those measured for the L5 chondrite, Knyahinya, whose pre-atmospheric radius of similar to 45 cm is thought to lead to the maximum possible production rates in chondrites. We infer that the pre-atmospheric radius of St-Robert was within 5 cm of the value that supports maximum production rates (i.e., 45 +/- 5 cm). This radius corresponds to a pre-atmospheric mass of (1.3 +/- 0.4) x 10(3) kg.The agreement of exposure ages for St-Robert obtained in several different ways and the similarity of the depth profiles for C-14, Al-26, Be-10, and Ne-21 argue against a lengthy pre-exposure of St-Robert on the parent body and against a two-stage exposure after launch from the parent body. Following Morbidelli and Gladman (1998), we suggest that St-Robert was chipped from deep in its parent body, spent the next 7-8 Ma without undergoing a major collision, was nudged gradually into an orbital resonance with Jupiter, and then traveled quickly to Earth.

We measured cosmogenic radionuclides (Be-10, Al-26, and Cl-36) and noble gases (He, Ne, and Ar) in 10 specimens of the Mocs L6 chondrite to determine the exposure history and preatmospheric relationship among fragments from known locations in the strewn field. Cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15.2 Ma. However, Mocs has very low Ne-22/Ne-21 ratios indicative of deep burial in a large meteoroid, but radionuclide levels at saturation values typical for much smaller meteoroids: this paradox suggests a possible complex exposure. For the latter case, we propose a two-stage exposure history in which Mocs initially was deeply buried in a large object for 110 Ma, followed by exposure in a 65 cm object for 10.5 Ma. Relative shielding was inferred from the measured 22Ne/21Ne ratios assuming constant Ne-22/Ne-21 production for all samples during the first stage. These shielding levels, which are supported by estimates based on Cl-36 production by neutron capture, indicate a possible relationship between depth of samples in the Mocs meteoroid and fall location in the strewn field.

Cosmic-ray produced nuclides were measured in samples from eight pieces of the L5-6 chondrite Mocs from known locations in the strewnfield. We measured '%e and %Al in the bulk phase along with 3 6 ~ 1 in both the metal and silicate phases. Relationships of the activities of these radionuclides from various meteorite pieces in the Mocs strewnfield provide new insight into the association of meteorite fragments to each other in the pre-atmospheric parent body. Results suggest a >2n irradiation for the Mocs meteoroid which was less than 1 meter in radius. Cosmogenic radionuclides are produced by the interaction of cosmic radiation with target elements in objects in space. The production of cosmogenic radionuclides has been the basis for the estimation of cosmic ray exposure (CRE) ages in small objects. These nuclides have also been used to determine the terrestrial ages of Antarctic meteorites. However, little work has been done on large meteorites, with one exception being the depth profile of the Canyon Diablo meteorite by Michlovich et al.[l] Measurements of cosmogenic radionuclides in individual meteorites has also been used to provide information on the geometry of meteoroids.[2] However, no similar data exist on the strewnfield of a meteorite shower. It is well-known that larger fragments tend to travel farther and impact at the far-end of the strewnfield. As a working hypothesis, we might expect a systematic variation of cosmogenic radionuclides with position of fragments in a strewnfield.

schutz, N. Bhandari, A. K. Singhvi, R. Hutchison, P. H. Benoit, D. W. G. Sears, L. Franke, P. Scherer, and L. Schultz, Purdue University Department of Chemistry (1393 Brown Building West Lafayette, IN 47907-1393 ferko@purdue.edu), Physical Research Laboratory (Ahmedabad, India), British Museum of Natural History (London, England), University of Arkansas Department of Chemistry and Biochemistry (Fayetteville, AR), MaxPlanck-Institut für Chemie (Mainz, Germany).