Separating thermal metamorphism from shock metamorphism in the L chondrite asteroidal parent body

The absence of a correlation between petrologic type and shock stage in L-group ordinary chondrites indicates that these metamorphic processes have produced distinct chemical and textural changes in this meteorite class. A petrologic and geochemical study of 63 L chondrites has shown that thermal metamorphism tends to homogenize the Ca component in matrix clinopyroxenes and produces normally zoned Fe-Ni metal and monocrystalline troilite, both of which indicate slow cooling from peak metamorphic temperatures. Shock metamorphism produces brittle deformation features in olivine and pyroxene and metal-sulfide textures, such as homogeneous taenite, polycrystalline kamacite, polycrystalline troilite, sheared troilite, and fizzed troilite, indicative of fast cooling from peak post-shock temperatures. Above shock stage S2 L chondrites may have experienced localized partial melting which produced a series of metal-sulfide melt compositions and melt drop sizes, which appear to correlate with shock stage. Three geothermometers were applied to a representative group of L chondrites to compare cation-exchange blocking temperatures with petrologic type and shock stage. Olivine + spinel temperatures were found to correlate with shock stage, explaining the extreme homogeneity of major element compositions in L chondrite matrix chromites. Results obtained from the two-pyroxene and metal + sulfide thermometers show an inverse correlation between peak thermal metamorphic temperature and petrologic type, suggesting that the L chondrite asteroid body had an onion-shell internal structure prior to impact disruption. Peak thermal metamorphic temperatures at the core of this body were determined by pyroxene thermometry to be -1273 K. Results of this research were combined with recent laboratory measurements of L chondrite physical and thermal properties to produce a revised model of an internally heated body in an attempt to understand the thermal evolution of the L chondrite parent body. With the use of the decay of ²⁶Al as a heat source, model calculations predict that the L chondrite asteroidal parent body accreted 2-4 Ma after Allende CAI formation as a small planetesimal (radius of -60 km) that was heated to peak temperatures of 1273 K in the core region, and then cooled slowly to the initial surface temperature of 180 K in -60 Ma. Major impact events at -4.2 Ga, and again at -0.5 Ga, formed the shock metamorphic features that now overprint thermal metamorphic features in L chondrites.