| Summary: | P(論文) ^<40>(Ar)-^<39>(Ar) ages have been determined for three chondrites (ALHA77015,77214,77304) from Allan Hills, Antarctica. Although younger ages have been observed at lower and higher temperatures for ALHA77015 (L3), the intermediate plateau-like age indicates a value of 4514±48 (1σ) Ma, though the plateau range is relatively narrow about 27% of the total ^<39>(Ar). ALHA77304 (LL3) shows a typical plateau age of 4503±52 Ma at higher temperatures, covering about 46% of the total ^<39>(Ar). Seriously weathered ALHA77214 (L or LL) exhibits a typical stair-case age pattern, increasing from about 1600 Ma up to about 4450 Ma. When coupled with reported data on other chondrites, there may be a rough correlation between the ^<40>(Ar)-^<39>(Ar) age and the degree of metamorphism defined from their petrologic type for unshocked L chondrites. However, such a trend is not clear for H and LL chondrites and a more systematic survey is required to settle the problem. departmental bulletin paper
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