Noble gases and mineralogy of primary igneous materials of the Yamato-793605 shergottite

Mineralogical and petrologic study on the Y-793605 lherzolitic shergottite using an optical microscope and an electron microprobe shows that this meteorite is a primary cumulate rock with close affinity to ALH77005 (and possibly to LEW88516), and is different from Shergotty. Strong shock effects. 30-50 GPa, are observed as finely fractured texture of pyroxene, maskelynite and brownish color olivine. Noble gas analyses by total melt and stepwise heating were performed on 17.86 and 50.99 mg primary igneous materials from Y-793605,10,respectively. Xe isotopic ratios show weak isotope signature for shergottite, which is different from those of nakhlites, Chassigny and ALH84001. Cosmic-ray exposure ages calculated from cosmogenic ^3He and ^Ne are 5.1-5.8 Ma, which is slightly longer than the ages, 3.3-4.0 Ma, reported for other two lherzolitic shergottites ALH77005 and LEW88516. Multistage exposure to cosmic-rays would be responsible for the difference. Apparent K-Ar age of 1430-1860 Ma is calculated, which is concordant with the young crystallization ages for martian meteorites, though the age is still ambiguous