Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..346e&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 346
Computer Science
1
Asteroid, 4 Vesta, Diogenites, Eucrites, Exposure Ages, Howardites, Noble Gases
Scientific paper
It is likely that the eucrites and their associates, the howardites and diogenites, sample the surface and shallow interior of a single parent body, possibly 4 Vesta (cf. [1] and [2]). A break-up event that reaches deep enough may, thus, eject asteroidal fragments representing meteorites from all three classes. In this work we present a comprehensive investigation of the exposure age clusters for howardites, eucrites, and diogenites (HEDs). Cosmic-ray exposure ages critically depend on the production rates for cosmic-ray produced nuclei. For eucrites shielding independent production rates for ^21Ne and ^38Ar have been determined previously [3,4]. We now present production rates of ^3He, ^21Ne, ^33Ar, ^78Kr, ^83Kr, and ^126Xe for eucrites, howardites, and diogenites as a function of shielding, where appropriate, and of target element abundances as derived on the basis of ^81Kr-Kr ages. E.g., for ^21Ne we obtain: P(sub)21 (EUC) = 8.43 P^1(sub)21 [16.1 (^22Ne/^21Ne)(sub)c - 10.3]^-1, P(sub)21 (HOW) = 6.16 P^1(sub)21 [18.1 (^22Ne/^21Ne)(sub)c - 14.1]^-1, P(sub)21 (DIO) = 4.81 P^1(sub)21 [25.7 (^22Ne/^21Ne)(sub)c - 23.7]^-1, where P^1(sub)21 = 1.63 [Mg] + 0.6 [Al] + 0.32 [Si] + 0.22 [S] + 0.07 [Ca] + 0.021 [Fe + Ni] as given by [3]. (Elemental abundance [x] in weight %, P(sub)21 in 10^10 cm^3 STP/g, Ma). Average cosmic-ray exposure ages were derived from as many nuclei as possible for 14 HEDs analyzed by us (see also [5,6]) and for those compiled by [7]. Two major exposure age clusters at 21 and 38 Ma are represented in all three meteorite classes (Fig. 1). In the cluster at 21 +- 4 Ma are 12 out of 39 eucrites, 6 out of 14 howardites, and 7out of 12 diogenites. In the cluster at 38 +- 8 Ma are 6 eucrites, 5 howardites, and 4 diogenites. A third common break-up event at 5 +- 1 Ma is indicated by the remaining diogenite, three eucrites, and one howardite. Schultz [8] found major clusters for eucrites at 13, 21, 26, and 40 Ma for howardites around 10 and 24 Ma, and for diogenites at 17 Ma, whereas the two clusters for diogenites calculated by [9] are in excellent agreement with our results. The cluster distribution observed in our work is consistent with the stratigraphy of 4 Vesta [10] and strengthens the view that this asteroid is the HED parent body: large collisional events liberate material representing all three classes, whereas smaller ones do not reach down to depths where diogenites can be ejected. Therefore, eucrites that are suggested to originate from the outer layer are more frequently expelled than diogenites and show additional exposure age clusters. Howardites are in all respects (mineralogy, chemistry, exposure age distribution) an intermediate case between eucrites and diogenites. Acknowledgement: This work was supported by the Swiss NSF. References: [1] McCord T. B. et al. (1970) Science, 168, 1445-1447. [2] Consolmagno G. J. and Drake M. (1977) GCA, 41, 1271-1282. [3] Schultz L. and Freundel M. (1985) In Isotopic Ratios in the Solar System, (CNES ed.), CEPAD, 27-33. [4] Freundel M. et al. (1986) GCA, 50, 2663-2673. [5] Michel Th. and Eugster O. (1989) Meteoritics, 24, 304. [6] Michel Th. et al. (1991) Meteoritics, 26, 372. [7] Schultz L. and Kruse H. (1992) A Data Compilation, MPI, Mainz. [8] Schultz L. (1987) LPS XVIII, 884-885. [9] Welten K. C. et al. (1991) Meteoritics, 26, 408. [10] Dreibus G. et al. (1977) Proc. LPSC 8th, 211-227.
Eugster Otto
Michel Th.
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