Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.487k&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 487
Other
1
Lithology, Meteoritic Composition, Radioactive Age Determination, Shock Wave Interaction, Cooling, Crystallization, Degassing, Temperature Effects, Thermal Stresses
Scientific paper
Padvarninkai was recently described as a polymict eucrite containing three different lithologies: Pad-1, fine- to coarse-grained eucritic clasts; Pad-2, a fine-grained lithology with quenched texture; and Pad-3, a partly glassy matrix between eucritic clasts. In addition to Pb-Pb studies on zircons from lithology Pad-1, we analyzed all three lithologies by high-resoluion stepwise heating Ar-40/Ar-39 dating to determine the postcrystallization thermal history of the meteorite. Pad-1 yields a well-defined plateau age of 3.80 b.y. for the high-temperture fractions. This age marks the time when the rock was totally degassed by, most likely, impact-induced reheating. Pad-3 yields a very similar degassing pattern, apparent age, and K/Ca spectra to Pad-1, but no well-defined high-temperature plateau. The apparent ages for the 1180-1530 C fractions increase monotonously from 3.44 to 3.71 b.y. Thus, the glassy matrix of Pad-3 was most likely formed 0.6 Ga by melting of Pad-1 like rock where also the unmolten eucritic lithology Pad-1 was affected. Pad-2 is less retentive than the two others. After correction for a trapped component the first 65% of the age spectrum has a plateau at 1.71 b.y. For the last 35% of the spectrum the ages increase up to approximately 3 b.y. Pad-2 contains maskelynite of bytownitic composition reflecting severe shock and a quenched texture indicating rapid cooling to subsolidus temperatures. The plateau age most likely dates these events. After 1.71 b.y. the K-Ar system remained completely closed. Therefore, it is impossible that lithology Pad-2 was in thermal contact with Pad-1 and Pad-3 since these lithologies were partially degassed as late as 0.6 Ga. Thus, 0.6 b.y. is an upper limit for the compaction age of the Padvarninkai eucrite.
Bukovanska M.
Jessberger Elmar K.
Kunz Jacob
Trieloff Mario
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