THE METAMORPHIC HISTORY OF EUCRITES AND EUCRITE-RELATED METEORITES AND THE CASE FOR LATE METAMORPHISM

We report induced thermoluminescence (TL) data for separates from thre e howardite, eucrite and diogenite (HED) meteorites and the Vaca Muert a mesosiderite. The results of thermal modeling of the surface of thei r parent body are also described. The TL sensitivities for matrix samp les from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regol iths. Within an isochemical series of HED meteorites, TL sensitivity r eflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 8 7509, 513 and 531 paired howardite primarily reflect differences in mi neralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrite s experienced a reheating event involving temperatures >800 degrees C, which is thought to have been due to impact heating, and therefore th at the event was concurrent with or postdated brecciation. The Vaca Mu erta clasts are essentially unmetamorphosed, but the induced TL data i ndicate that the remaining howardite, eucrite, diogenite and mesosider ite (HEDM) meteorites experienced metamorphism to a variety of intensi ties but involving temperatures <800 degrees C. Laboratory heating exp eriments show that temperatures >800 degrees C cause a change in TL pe ak temperature. Feldspars from a variety of terrestrial and extraterre strial sources show this behavior, and x-ray diffraction and kinetic s tudies suggest that it is indirectly related to AI,Si disordering. Coo ling rates are not consistent with autometamorphism following the init ial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models sugge st that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism co rresponding to different thicknesses of blanket, between essentially 0 and similar to 2 km, rather than different burial depths in a regolit h 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.