Irradiations ancienne et recente des aubrites

Abstract

Three gas-rich aubrites (Bustee, Khor Temiki and Staroe Pesyanoe) have been studied and compared with five other non gas-rich aubrites (Aubres, Bishopville, Cumberland Falls, Norton County and Shallowater), by means of the track method. The gas-rich objects show a strong correlation between the percentage of solar flare irradiated crystals (track-rich crystals, C.R.T.) and the solar wind trapped 36A concentrations. A similar trend (though not identical) is observed between 36A and C.R.T. in many lunar soils. Furthermore, by considering in each of the 8 aubrites the track production rate/my attributable to the ‘exposure age’ of the meteoroids, the following objects (in our experimental conditions), Staroe Pesyanoe, Khor Temiki and Norton County, show a track density gradient background clearly attributable to irradiation by galactic cosmic rays. The minimum track production rate in aubrites, excepting Shallowater, show very low values (< 0.0002 − 8.8 × 10 4 cm −2 my −1) suggesting either high ablation/fragmentation during the atmospheric entry or an irradiation stage at great depth (> 300 g cm −2) in the parent body. The latter suggestion seems applicable to objects like Cumberland Falls aubritic portion and Norton County. Preirradiation stages on a parent body may account for both the surface exposure to the solar wind and solar flare irradiated crystals, along with the excess neutron flux as observed in Norton County, if the new ‘exposure age’ value (∼ 90 my) is considered. In such a case, a very efficient turnover had to be acting at the time the preirradiation occurred. Among aubrites, Shallowater appears to be a very peculiar object on petrographical, chemical, trapped noble gases and exposure ages grounds. The Shallowater sample we studied shows a very high track production rate: 26 × 10 4 cm −2 my −1 which can be well explained on account of the meteoroid exposure age, either on the basis of cosmogenic rare gas or cosmic ray track data.