Solar wind and cosmic ray exposure history of lunar meteoriteYamato-793274

• Main Authors: Eugster,O., Michel,Th., Niedermann,S.
• Format: Report
• Language: English
• Published: Physikalisches Institute, University of Bern/Physikalisches Institute,University of Bern/Physikalisches Institute, University of Bern 1992
• Subjects: Yamato; Antarc*; Antarctica
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=4569; http://id.nii.ac.jp/1291/00004569/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=4569&item_no=1&attribute_id=18&file_no=1

We measured the isotopic abundances of the noble gases in bulk samples and grain-size separates of lunar meteorite Yamato-793274. The presence of relatively high contents of trapped solar wind gases and stable cosmic-ray produced nuclei indicates that Y-793274 consists of submature lunar regolith material, that is, Y-793274 is not quite as rich in solar wind gases as mature lunar soil. The K-^<40>Ar gas retention age is 2800±400 Ma. Breccia formation occurred 500-1000 Ma ago as estimated from a trapped ratio ^<40>Ar/^<36>Ar=2.36±0.10. The average shielding depth of 35±15g/cm^2 during the total exposure to cosmic rays was derived from the depth sensitive cosmogenic ratio ^<131>Xe/^<126>Xe=4.1±0.7. The total exposure to cosmic rays in the lunar regolith lasted for 700±200 Ma and most of this exposure occurred before breccia formation. The concentration of ^<81>Kr is far below the 4π-saturation concentration in space; we calculate an upper limit of 0.12 Ma for the duration of the Moon-Earth transfer. The history of exposure to cosmic rays and solar wind particles for Y-793274 is very similar to that for another lunar meteorite, ALHA81005. Because the recovery sites in Antarctica lie 3000km apart, pairing is not possible. However, the two meteorites may originate from the same ejection event on the Moon although they are mineralogically different and an inhomogeneous ejection site would have to be assumed.