Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.227g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 227
Astronomy and Astrophysics
Astrophysics
3
Scientific paper
Enstatite achondrites display long exposure ages (T(sub)e) compared to the collisional lifetimes of ordinary chondrites of ~17 Ma (Eberhardt et al., 1965; Graf and Marti, 1992), and therefore suggest special orbits for these meteorites. Recently, Gaffey et al. (1992) suggested a link between aubrites, near- Earth asteroid 3103 and E-type asteroids of the Hungaria family, based on spectral matches and orbital considerations. This would represent the first direct link of a meteorite type and an asteroid family. Moreover, the orbital elements of Apollo object 3103 are consistent with long collisional lifetimes as observed for the Norton County and Mayo Belwa aubrites (T(sub)e = 110-120 Ma). There has been much debate over the genetic relation between E-chondrites and aubrites (Keil, 1989). Crabb and Anders (1981) found that exposure ages show a pronounced trend E4 < E6 < aubrites. We reevaluate the cosmic ray record of enstatite meteorites based on an updated database (Schultz and Kruse, 1989) as well as improved production rates and shielding corrections. We make the following observations: 1) Five out of 11 aubrites cluster at T(sub)e = 55+-5 Ma. All three known solar gas-bearing aubrites belong to this group, lending considerable support to the reality of a discrete event at this time. 2) While Gaffey et al. (1992) suggested that a significant fraction of aubrites may derive from 1-2 meteorite streams, the average ^22Ne/21Ne ratios of ~1.11 and saturated ^26Al activities render a long 2-pi irradiation interval preceding the (short) 4- pi exposure in space unlikely. 3) In a ^38Ar/^21Ne vs ^22Ne/^21Ne diagram, the inferred spallation ratios of EH-chondrites plot systematically below the expected correlation line. However, Cl abundances in EHs are about an order of magnitude larger than those in ordinary chondrites. If significant amounts of ^36Ar produced by the reaction ^35Cl(n,gamma)^36Cl --> ^36Ar are present, the standard procedure overestimates trapped Ar. Indeed, ^36Ar/^38Ar ratios of ~7 were observed in stepped heating experiments of Abee separates, indicating the presence of such a component (Wacker, 1982). We calculate a ratio of ^36Ar(n,gamma)/^36Ar(sp) of 1.7- 2.6 from the observed shifts in the 3-isotope plot. Since concentrations of (n,gamma) products are predicted to increase with shielding more steeply than spallation products, it is surprising that the calculated ^36Ar(n,gamma)/^36Ar(sp) ratios are rather constant over a wide range of shielding conditions. Alternatively, matrix effects (enhanced cascade of secondary GCR particles in high Z targets), previously suggested for mesosiderites (Begemann and Schultz, 1988) are also evaluated. 4) Five out of 11 EH-chondrites have exposure ages <4Ma. This indicates either a recent break-up or very short lifetimes. In contrast, 6 of 10 ELs cluster at T(sub)e = 30+-3 Ma, indicating a collisional event at this time. Only one EL chondrite has an exposure age <27 Ma. The exposure age systematics show that discrete collisional events are at least as common for enstatite meteorites as for ordinary chondrites (Marti and Graf, 1992). The systematics do not suggest an origin off the same immediate parent object. However, differential orbital evolution following an early catastrophic disruption of a parent asteroid cannot presently be discounted, except that the solar wind loading of aubrites requires the evolution of a regolith. The high retention of radiogenic ^40Ar(sub)r rules against major heat pulses in the recent history. References Begemann F. and Schultz L. (1988) Lunar Planet. Sci. (abstract) 19, 51-52. Crabb J. and Anders E. (1981) Geochim. Cosmochim. Acta 45, 2443-2464. Eberhardt P., Eugster O., and Geiss J. (1965) J. Geophys. Res. 70, 4427-4434. Gaffey M. J., Reed K. L., and Kelley M. S. (1992) Lunar Planet. Sci. (abstract) 23, 395-396. Graf Th. and Marti K. (1992) Lunar Planet Sci. (abstract) 23, 433-434. Keil K. (1989) Meteoritics 24, 195-208. Marti K. and Graf Th. (1992) Ann. Rev. Earth Planet. Sci. 20, 221-243. Schultz L. and Kruse H. (1989) Meteoritics 24, 155-172. Wacker J. F. (1982) Ph.D. Thesis, Univ. of Arizona.
Graf Th.
Marti Kurt
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