[Show abstract][Hide abstract] ABSTRACT: Abstract— 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 22Ne/21Ne 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 ∼40 cm and are generally larger than depths estimated from published 60Co activities, perhaps because the track production rates adopted are too high. Depth profiles of the production rates of 14C, 36Cl, 26Al, 10Be, and 21Ne 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 ∼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) × 103 kg.
The agreement of exposure ages for St-Robert obtained in several different ways and the similarity of the depth profiles for 14C, 26Al, 10Be, and 21Ne 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.
[Show abstract][Hide abstract] ABSTRACT: By using accelerator mass spectrometry we have measured the 10Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the 10Be concentrations (106 atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured 10Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest 10Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The 10Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of 10Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-10Be material farthest—to Australia—while keeping the deeper-lying, low-10Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation.