Collisional records in LL‐chondrites

Abstract

One‐third of all the LL‐chondrites have exposure ages of ∼15 Ma and were exposed to cosmic rays following a collisional break‐up. The probability that the 15‐Ma peak represents a random signal is calculated to be less than 2%. Considerably lower probabilities are obtained if only LL5s or subgroups of high 40Ar retention are used. Furthermore, we show that the peak shape agrees with statistical constraints obtained from multiple analyses of samples from the St. Severin LL6‐chondrite. The frequency in and out of the 15‐Ma peak varies significantly for different petrographic LL‐types. The radiogenic 40Ar retention systematics (most LL‐chondrites retained 40Arrad) shows that no substantial heat pulse resulted in the 15‐Ma collisional event. Interestingly, smaller exposure age clusters at ∼28 Ma and ∼40 Ma match up well with clusters in the histogram of L‐chondrites. The distribution of LL‐exposure ages is not consistent with that expected for a quasi‐continuous injection of LL material into a resonance zone of the asteroid belt. The near absence of exposure ages shorter than 8 Ma either indicates a lack of recent collisional events or considerably longer transfer times than inferred from dynamical considerations.

Cosmic‐ray exposure ages of diogenites and the recent collisional history of the howardite, eucrite and diogenite parent body/bodies

“…A cluster possibly represents a single large event, but with the uncertainties given, two closely spaced events cannot be ruled out. In order to identify a peak in the exposureage distribution as a potential single impact event, we compared the peak width with the overall analytical uncertainties and calculated the random probability that such a peak would occur in a continuous distribution (Graf and Marti, 1994). …”

“…However, these ages have much larger uncertainties than our diogenite ages, because: (1) the use of many old noble gas analyses introduces considerable interlaboratory bias (Graf and Marti, 1994), (2) the lack of chemical analyses for most eucrites and howardites hampers the calculation of accurate production rates, especially for P38, (3) no shielding corrections were made for P38, (4) shielding corrections for P21 were made on the basis of the 22Nd1Ne ratio, which is not very reliable due to the low Mg/Si ratios of howardites (0.35) and eucrites (0.20) (Cressy and Bogard, 1976) and (5) variable losses of cosmogenic Ne from the feldspar fraction of eucrites and howardites (Megrue, 1966) lead to underestimation of the 21Ne age. This explains why only about one-half of the 21Ne and 38Ar ages agree within 25%, whereas some of them even differ by a factor of 2 or more.…”

“…This uncertainty is estimated at 5% for multiple samples of a single meteorite, but interlaboratory bias, the use of less precise analytical data and limitations in the shielding corrections for meteorites of different sizes and shapes may increase the peak width of an exposure-age cluster to 9-10% (Graf and Marti, 1994). However, these effects are expected to be small in our data set of diogenite ages, since (1) the only exposure age (Manegaon, 6.5 Ma) based on old noble gas analyses does not contribute to any peak, (2) interlaboratory bias is largely avoided because all ages except for Ibbenbuhren and Manegaon are based on noble gas analyses from one laboratory (i.e., the MPI in Mainz), (3) Table 4).…”

“…To this end, we assumed a "continuous" exposure-age distribution based on a constant production of collisional debris (NO) and mean orbital lifetimes ( t l ) of 1&20 Ma, as proposed by Graf and Marti for ordinary chondrites (1994). Contrary to their approach, we did consider age intervals of 0-5 Ma and ages 250 Ma, because 12% of all ordinary chondrites have exposure ages <S Ma and -4% of the ordinary chondrite ages are >50 Ma (Marti and Graf, 1992;Graf and Marti, 1994). A priori, it is not clear that the "continuous" distribution for ordinary chondrites is the same as for HED meteorites, but the overall shape of the ordinary chondrite age distribution is similar to the one for the HED meteorites, with median ages of -20 Ma, maximum ages of -80 Ma and a relative lack of ages 4 0 Ma.…”

Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

“…In such a scenario, the different petrologic types would have formed on separate parent bodies (Yomogida & Matsui 1984), which is in contradiction with the cosmic-ray exposure (CRE) ages of individual OCs. The latter shows that, for some classes (mainly H chondrites), several petrologic types (mainly types 4 and 5 in the H case; Marti & Graf 1992;Graf & Marti 1994;Graf & Marti 1995) show similar peaks in their CRE age distribution, and thus suggest that the different petrologic types do originate from the same parent body.…”