The metamorphic history of eucrites and eucrite-related meteorites and the case for late metamorphism

Details The metamorphic history of eucrites and eucrite-related meteorites and the case for late metamorphism The metamorphic history of eucrites and eucrite-related meteorites and the case for late metamorphism

Nov 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..917s&link_type=abstract

Meteoritics & Planetary Science, vol. 32, no. 6, pages 917-927

Computer Science

7

Scientific paper

We report induced thermoluminescence data for separates from three HED meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures >800oC, thought to have been due to impact heating, and therefore that the event was concurrent with or post-dated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining HEDM meteorites experienced metamorphism to a variety of intensities but involving temperatures <800oC. Laboratory heating experiments show that temperatures >800oC cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and X-ray diffraction and kinetic studies suggest that it is indirectly related to Al,Si disordering. Cooling rates are not consistent with autometamorphism following the initial igneous event, or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially zero and about 2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar-Ar system for the HED meteorites.

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