Thermoluminescence Constraints on the Metamorphic, Shock, and Brecciation History of Basaltic Meteorites

by Batchelor, J. D.; Sears, D. W. G.

Induced thermoluminescence measurements have been performed on eighteen eucrites, thirteen howardites (plus seven splits from the Bholghati howardite), six diogenites, and fifteen mesosiderites in order to help clarify their metamorphic, shock, and brecciation history. The TL sensitivities of the basaltic meteorites reflect their bulk composition, and the effects of (1) igneous processes (diogenites and mesosiderites) and (2) the mixing of materials of different compositions (e.g., diogenitic and eucritic) during brecciation (howardites) can be observed. Two previously unclassified mesosiderites, LEW87006 and EET87500, are assigned to class A and B, respectively, on the basis of their TL sensitivities. The eucrites show a 15-fold range of TL sensitivities, which correlate with petrographic indicators of metamorphic intensity. We therefore describe a petrologic type scale based on TL and petrographic evidence, where the least metamorphosed eucrites are type 1 and most equilibrated are type 8. We see no evidence for two discrete metamorphic environments corresponding to the equilibrated and unequilibrated eucrites. The temperature of the dominant TL peak observed for basaltic meteorites, and experiments in which four eucrites with diverse petrographic properties were annealed at various temperatures in the laboratory, suggests the metamorphic equilibration temperatures for most basaltic meteorites were less-than-or-equal-to 800-degrees-C. This estimate is generally consistent with palaeotemperatures determined from two-pyroxene, olivine-pyroxene, and oxygen isotope equilibria. Assuming this temperature was typical of conditions during metamorphism, then burial depths for type > 5 eucrites were > 350 m and < 50 m for type 2 eucrites. An important exception is LEW85303 (and three samples paired with it, LEW85300, 85302, and 88005) which has been shock-heated above 1000-degrees-C. Ibitira and LEW85305 have also been shocked but subsequently suffered low-temperature annealing (less-than-or-equal-to 800-degrees-C). Since TL peak temperatures are related to the degree of disorder in the Al, Si chain in feldspar, the present data provide independent evidence for very slow cooling rates for mesosiderites and for the slower cooling rates for some cumulate eucrites relative to equilibrated non-cumulate eucrites.

Geochimica Et Cosmochimica Acta
Start Page
1872-9533; 0016-7037