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

Author

Marvin Edward Bennett

Date of Award

5-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Harry Y. McSween Jr.

Committee Members

Theodora C. Labotka, Lawrence A. Taylor, T. Ffrancon Williams

Abstract

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.

Recommended Citation

Bennett, Marvin Edward, "Separating thermal metamorphism from shock metamorphism in the L chondrite asteroidal parent body. " PhD diss., University of Tennessee, 1995.
https://trace.tennessee.edu/utk_graddiss/9941