Characterization of Ar-degassing from Antarctic meteorites

• Main Author: Ichiro Kaneoka
• Format: Report
• Language: English
• Published: Geophysical Institute, Faculty of Science, University of Tokyo 1984
• Subjects: Antarctic; The Antarctic; Antarc*; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1731; http://id.nii.ac.jp/1291/00001731/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1731&item_no=1&attribute_id=18&file_no=1

Based on the Ar-degassing patterns from the Antarctic meteorites, it is intended to find some relationships between the types of Ar-degassing and the states of Antarctic meteorites. For this purpose, diffusion parameters (D/α^2,E) were obtained for each meteorite by using the ^<39>(Ar)-degassing pattern which had been obtained when those meteorites had been dated by the ^<40>(Ar)-^<39>(Ar) method. The diffusion constants (D/α^2) are estimated to be (0.2-3)×(10)^<-5> s^<-1> at 1000℃. The activation energies (E) range from about 10 to 50 kcal/mole, but mostly 20 to 40 kcal/mole. Nonequilibrated chondrites seem to have lower activation energies than equilibrated chondrites. On the basis of integrated fraction of Ar, a diagram is exploited to compare relative degassing patterns of Ar between neutron induced K-derived ^<39>(Ar) and the others (^<36>(Ar), ^<37>(Ar), ^<40>(Ar)). According to the Ar-degassing patterns in this diagram, the Antarctic meteorites are classified into three Types, A, B and C. It is found that achondrites are generally classified into Type A, where ^<40>(Ar) correlates well with ^<39>(Ar) and ^<36>(Ar) (and ^<37>(Ar)) degass at slightly higher temperatures than ^<39>(Ar). Chondrites are classified into Type B, where ^<40>(Ar) correlates with ^<39>(Ar), but ^<36>(Ar) (and ^<37>(Ar)) degass at definitely higher temperatures than ^<39>(Ar). Weathered achondrites and chondrites are generally grouped into Type C, where ^<40>(Ar) does not correlate with ^<39>(Ar) and degassing patterns of ^<36>(Ar) and ^<37>(Ar) are also non-systematic. Thus, such classification would give another measure to evaluate the reliability of an ^<40>(Ar)-^<39>(Ar) age obtained in addition to conventional criteria.