Chemical and Physical Studies of Type-3 Chondrites .11. Metamorphism, Pairing, and Brecciation of Ordinary Chondrites

by Sears, D. W. G.; Hasan, F. A.; Batchelor, J. D.; Lu, J.

Induced thermoluminescence (TL) data for 69 type 3 ordinary chondrites have been obtained. With updated literature data, this brings to 125 the number of type 3 chondrites for which TL data are available, probably around 106 when pairing among Antarctic samples is taken into account. The data are used, with literature data for olivine heterogeneity, carbon content, and inert-gas content, to assign the meteorites to petrographic types. Brecciation is common among type 3 ordinary chondrites, and petrographic and inert-gas data indicate that several are regolith breccias. However, it is unusual for clasts from a single breccia to differ by more than 0.4 in petrologic type. In agreement with earlier work, the new data produce two clusters on a TL peak temperature vs. TL peak width plot, with type 3.2-3.4 tending to plot in one cluster and type 3.6-3.9 in the other. The implications of this clustering for paleotemperatures have been previously discussed. and recent data from other paleothermometry techniques generally confirm the temperatures of 500-degrees-600-degrees-C for type 3.5 suggested by the TL data. H3 chondrites show higher TL sensitivities than L3 and LL3 chondrites, so that most of the type <3.4 samples are L or LL. chondrites, while, except for two splits from the ALHA81251 breccia (one of 3.2 and one of type 3.4), the most primitive H chondrite is type 3.5. This trend is probably not due to different accretion times or burial depths, but is due to differences in the physical properties of the three classes. Differences in physical properties may be more important than accretion times and burial depths in determining the thermal history of chondrites.

Proceedings of Lunar and Planetary Science
Start Page