Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.245k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 245
Mathematics
Logic
1
Scientific paper
Eucrites are one of the three classes of achondrites which are supposed to have originated from the same celestial body, the HED (Howardite-Eucrite-Diogenite) parent body. These achondrites provide evidence that at least one asteroid experienced extensive differentiation and basaltic volcanism during the early stage of the solar system. Sm-Nd ages of about 4.5 Ga most probably date the crystallization of the crust of the parent body (Warren et al., 1991). ^40Ar-^39Ar ages of Eucrites (Podosek and Huneke, 1973; Bogard and Garrison, 1991) range between 3.4 Ga and 4.4 Ga and cluster at about 3.5 Ga and 4.0 Ga (Bogard and Garrison, 1991). Thus they significantly postdate the crystallization and identify--possibly repeated--secondary processes such as impact induced brecciation, reheating and recrystallization. The K-Ar- system can rather easily be disturbed or reset at conditions which do not necessarily manifest themselves in mineralogical or petrographic alterations. It is expected that by high resolution ^40Ar-^39Ar analysis of HED achondrites the duration of the intense bombardment of the HED parent body can be obtained. Here we report on a high resolution ^40Ar-^39Ar study of matrix samples and individual clasts from four Eucrites: Stannern, Pasamonte, ALH76005, and ALH80102. All age spectra are complex: In general the apparent ages increase with increasing degassing temperature and in cases are additionally disturbed by ^39Ar recoil. Another general feature are the K/Ca spectra which for all samples are almost constant for the first 70% of ^39Ar release and then level off by about one order of magnitude. The decrease of K/Ca at high extraction temperatures is accompanied by an increase of the apparent ages. Thus, in general, the high K/Ca portions of the samples are younger and the apparent ages provide an upper limit to the last thermal/shock event, while the low K/Ca portions yield lower limits to the time of the last total degassing of the rocks. Stannern: The age spectra obtained on three samples taken from this monomict breccia are distinctly disturbed by ^39Ar recoil, rather independent of grain size and plagioclase enrichment. Most probably submicroscopic pyroxene needles and grains inside the plagioclase, which are often observed in eucritic feldspar (Harlow and Klimentidis, 1980), serve as the ^39Ar receptors. The last degassing event happened 3.5 +- 0.1 Ga ago and was almost complete. The fine and coarse grained portions--i.e., the K- bearing feldspar therein--reacted very similar to that event. Pasamonte: The age spectra of the three clasts with different petrographic features taken from this polymict breccia have in common high temperature ages of 4.4-4.5 Ga similar to the crystallization age. The apparent ages of the three clasts, however, differ at lower extraction temperatures indicating either three independent partial degassing events (at 4.1 Ga, 4.0 Ga and 3.9 Ga) or different reactions on one event which happened 3.9 Ga ago (or later). ALH76005: The spectra obtained from two clasts from this polymict breccia are similar in shape but differ in their absolute age values. The two lower age limits are 3.0 Ga and 3.3 Ga for clast 1 and clast 2, respectively. Both spectra exhibit distinctly different intermediate plateau (given the small uncertainties and the expansion of the scale) at 3.7 Ga and 3.5 Ga for clast 1 and clast 2, respectively. ALH80102: Except for the very last extraction steps, the age and K/Ca spectra of the two clasts from this polymict breccia are almost identical. Most probably Ar loss occurred 2.8 Ga ago (or later) from rock that possibly was subjected to a prior degassing event at around 3.8 Ga. All in all, the above presented results support a period of cataclysmic bombardment on the HED parent body combined with total resetting of the K-Ar-systems 4.5-3.9 Ga ago which was followed by a period of less intense impact induced metamorphism causing partial loss of radiogenic ^40Ar 3.9-2.8 Ga ago. References: (1) Warren, P.H., Haack, H., and Rasmussen, K.L. (1991) J. Geophys. Res. 96. B4. 5909-5923. (2) Podosek, F.A. and Huneke, J.C. (1973) Geochim. Cosmochim. Acta 37. 667-684. (3) Bogard, D. and Garrison, D.H. (1991) Met. (abstract) 26. 320. (4) Harlow, G.E. and Klimentidis, R. (1980) Proc. Lunar. Planet. Sci. Conf. 11th. 1131-1143.
Jessberger Elmar K.
Kunz Jacob
Trieloff Mario
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