On the origin of hypersthene chondrites: Ages and shock effects of black chondrites

Abstract

In an attempt to establish the cause of the short gas-retention ages among the hypersthene chondrites, two lines of evidence were examined. (1) Rare-gas contents were determined in 20 black hypersthene chondrites by mass spectrometry. (2) Symptoms of shock and reheating were studied by X-ray diffraction analysis in olivine from 40 chondrites, including 15 black ones; and by metallographic techniques in 20 chondrites, including 18 black ones. On the basis of the results, it is concluded that an earlier suggestion by Anders that black chondrites are severely shocked is essentially correct. At least 15 black hypersthene chondrites seem to have suffered shocks ranging from a few hundred kilobars up to perhaps 1 or 1.5 Mbar. The shock apparently transformed olivine to a fine-grained polycrystalline state; the crystallites show a range of preferred orientation. From the cosmogenic He 3 Ne 21 ratios, it is concluded that black hypersthene chondrites generally suffered He losses of approximately 20–25% at a late stage, during the cosmic-ray bombardment era. The corrected He 3 and He 4 contents of 12 black hypersthene chondrites define an isochron of 520 ± 60 Myr with a (He 4/He 3) cosmogenic production ratio of 5.2 ± 0.3. A review of literature data shows that on the basis of their He contents, one-third of the hypersthene chondrites seem to have been definitely involved in the 520 Myr event, another one-third probably so. The large proportion of concordant K-Ar ages among the former strongly supports the contention that the 520 Myr date represents a genuine out-gassing event. These results bear out earlier conclusions by Anders that most, if not all the hypersthenes were on one or at most two parent bodies until relatively late in their history; and by Kirsten, Krankowsky, and Zähringer that short gas-retention ages of chondrites have arisen from collisional reheating of cold parent objects. The 520 Myrcollision seems to have involved the breakup of a parent asteroid of the hypersthene chondrites. The well-known paradox of the cosmic-ray exposure ages being much shorter (0.03 ∼- 60 Myr) can be solved by the hypothesis that kilometer-sized fragments of the parent asteroid were deflected by Mars into Earth-crossing orbits on a tume scale of ≈ 10 9 yr. Chondrites spalled off from such fragments by secondary collisions have lifetimes against planetary capture of 10 6–10 7 yr. The destruction lifetime of chondrites in the asteroid belt can be increased from 10 7 to ≈ 10 8 yr with a 10-fold reduction on the cross-section requirement.