Exposure history of the St‐Robert (H5) fall

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.

An entry model for the Tagish Lake fireball using seismic, satellite and infrasound records

“…The results of the St-Robert fireball/meteorite fall (Brown et al, 1996;Hildebrand et al, 1997) having both cosmogenic nuclide activities from recovered meteorites (which constrained the entry mass (Leya et al, 2001)) and satellite data available, yield an apparent integral luminous efficiency (Ti) of -1 0% in the silicon pass band ofthe satellite sensor for this H chondrite. This corresponds to the fraction ofthe total initial kinetic energy converted to radiation in this pass band.…”

Noble gas and oxygen isotopic compositions of Towada H6-chondrite from Aomori Prefecture, Japan

“…The low ( 3 He/ 21 Ne) c value reflects selective loss of 3 He compared to the Ne-loss. Since about half of the 3 He is produced by beta decay of the radioactive precursor nuclide 3 H, as pointed at by Leya et al (2001b), the lowered ( 3 He/ 21 Ne) c ratio may be partly due to 3 H loss before its radioactive decay (T 1/2 = 12.33 yr). The 3 H loss has been explained by higher diffusion rates of 3 H in metal than in silicate minerals (Leya et al, 2001a).…”

Géochimie isotopique Re-Os et Pb-Pb :

“…Studies by Wilson, (1994), and Leya et al (2001) have concluded that : a) it is a monomict breccia of a H5 ordinary chondrite with S2-S3 shock levels; b) it has a high porosity and a high permeability when compared to most ordinary chondrites; c) the well preserved fusion crust is -50% more permeable than a freshly cut surface; d) it was launched from deep in its parent body about 7-8 Ma ago, without undergoing further major collision, entered in orbital resonance with Jupiter and then traveled to the Earth; e) the modelled orbit is consistent with an aphelion located within the asteroid belt; and f) the pre-atmospheric mass was between 1 and 2 tons at an entry velocity of-13 km/s.…”

“…This rapid hitemperature oxidation could affect rhenium and osmium concentrations: the volatile nature of these two elements, when in oxide form, might favour a loss. Moreover, the high permeability of this crust (Leya et al 2001) might facilitate mobility of gases within it (both osmium and rhenium form volatile oxides at high fO2). Os values but are amongst the closest to the IIIAB isochron data; in fact Cr-2 is the most concordant datum (-0.5 delta unit, see Table 3).…”

“…Compositionally, the St-Robert meteorite is an unremarkable object, typical of a large group of H chondrites which were produced in a major collision 7 to 8 Ma ago (Leya et al 2001). However, it is one of a few where the simple post-collision history is well documented, the orbital parameters well-established and the recovery occurred within days after the fall event.…”