Statistics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.540m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 540
Statistics
Exposure Ages, Isotopes, Cosmogenic, Meteorites, Iron, Noble Gases
Scientific paper
The asteroid belt is considered to be the ultimate source of iron meteorites and it would be of considerable interest to obtain a chronology of break-ups of asteroidal objects. However, as multiple fragmentation of such objects did likely occur, the exposure ages date the break-off of iron masses from shielded locations within the immediate parent object. Meteorites which were fragmented in more than one collisional event may have recorded integral effects of cosmic ray interactions in varying geometrical configuration and individual stages may be difficult to unravel; we term such exposure histories "complex". Exposure age histograms based on potassium ages have been discussed by Voshage [1] and he concluded that irons of groups IIIA and IIIB reveal similar histograms and probably were derived from the same parent body. He also noted a cluster for group IVA members ,but no clear evidence for other clusters. We present the collisional evidence based on published noble gas data, coupled to the new production rates which we calculate for central locations, adjusted for off-center locations whenever concentration profiles can be inferred. Unlike potassium ages which show large uncertainties for ages < 300 Ma, T38 ages can be obtained for all iron meteorites. We note, however,that T38 values of five "old" irons are systematically 15% lower than potassium ages. We confirm the evidence for stochastic events for IIIAB and IVA irons. The statistics are improved because of the larger data base. There are interesting clusters also among ages < 100 Ma, in the range which overlaps the histograms of chondrites. Recent reports [2,3] of H-chondritic inclusions in IIE irons, whose exposure ages are consistent with H-chondrite clusters, point to a genetic link. Group IIAB reveals two clusters with T38 < 100 Ma, and both events appear to involve also IIE irons. Clusterings of two thirds of group IIIE members and of group IID irons appear significant. The youngest IVB ages coincide with the IVA peak at 380 Ma. The fitting algorithm used in our calibrations requires that non-complex Ar- ages and K-ages on average are the same. The ratios of the two ages range from 0.85 to 1.15. This variation roughly corresponds to the uncertainties in measured M values and in Ar concentrations. References: [1] Voshage H. (1978) EPSL, 40, 83-90. [2] Olsen E. et al. (1994) Meteoritics, 29, 200. [3] Casanova I. et al. (1995) Science, 268, 540.
Jeannot J.-P.
Lavielle Bernard
Marti Kurt
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