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Ordinary chondrites are the most common type of meteorite to fall to Earth and are composed of lithified primitive nebular materials which have experienced variable extents of thermal metamorphism and shock processing. They were subjected to radiogenic heating by incorporation of unstable short lived radionuclides such as 26Al in the early solar system. The relationship between metamorphism and impact processing in ordinary chondrites is not fully understood. An unresolved issue in the study of ordinary chondrites is whether their original parent bodies were fragmented by impacts into rubble-pile bodies while they were still hot, or whether they retained their onion-shell structures until they had shed their radiogenic heat. Heat is lost more quickly due to catastrophic impacts because warm material from the interior is exposed directly to the space environment until the impact debris re-accretes into a rubble-pile body, and is then distributed evenly between the surface and the interior of the new rubble-pile body. The extent of retrograde metamorphism possible in ordinary chondrites would therefore largely be dictated by the extent to which their parent bodies were broken up by impacts. Disaggregation caused by an impact would record fast cooling between the temperature at the time of breakup and the temperature at the time of re-accretion. In this thesis project, five H6 chondrites (Butsura, Estacado, Kernouve, Portales Valley, Queen's Mercy) and five L6 chondrites (Bruderheim, Holbrook, Leedey, Morrow County, Park) were subjected to three different thermometry analyses (pyroxene, olivine spinel, and metallographic) to determine their cooling profiles and evaluate same set of samples. Cooling rates for pyroxene and olivine--spinel thermometry systems are determined using the formulation of Dodson (1973) as modified by Ganguly & Tirone (1999). Cooling rates for the metallographic system are determined using the method developed by Wood (1967) as modified by Willis & Goldstein (1981). At temperatures higher than ~600 degrees C, all samples experienced cooling rates which are orders of magnitude faster (100's to 1000's of degrees C/kyr) than what is predicted for onion--shell thermal evolution (10's of degrees C/Myr) by e.g. Monnereau et al. (2013). At temperatures below ~600 degrees C, i.e. those recorded by the metals, cooling rates are much slower in comparison to the silicate/oxide systems, with the exception of Park, which continued to cool quickly. The discrepancy between high-- and low--temperature cooling rates for both H-- and L--chondrites can best be accounted for by a catastrophic impact which occurred while each body was still near its peak metamorphic temperature, followed by re--accretion into a rubble--pile, which would then cool slowly due to the poor thermal conductivity of rubble--piles. Shock heating does not appear to affect silicate--oxide thermometers. |
