The Jurassic Meteorite Flux: A Record from Extraterrestrial Chrome-Spinels

Caplan, Caroline E.
Huss, Gary R.
Geology and Geophysics
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This dissertation focuses on the classification of extraterrestrial chrome-spinels to determine meteorite types and fluxes during the Jurassic time period and the techniques needed to achieve such classifications. We know the relative abundances of meteorite types falling on Earth today, but we do not know what fell in the distant past. Abundances of the past are unknown because meteorites tend to only survive on Earth’s surface for a few tens of thousands of years due to the weathering environment of the Earth. Fortunately, chrome-spinels from disaggregated meteorites and micrometeorites can be preserved in limestone and retain their characteristic compositions. In many instances, the parent meteorite type of each grain can be determined by comparing their chemical compositions and oxygen isotope abundances to those of chrome-spinels from modern day meteorites. The goal of this dissertation is to classify chrome-spinel grains from the Jurassic period, determine the relative abundances of parent meteorite types, and compare these abundances to those of other time periods. This was achieved by ensuring we had a well-characterized database of chrome-spinel compositions from modern meteorites in order to reliably classify each remnant chrome-spinel. We also had to confirm that our chemistry and oxygen-isotope measurements of the Jurassic chrome-spinels were reliable. This was determined, in part, by taking new measurements of chrome-spinels from modern meteorites using the same electron and ion microprobes used for the remnant grains. It was also necessary to understand instrumental artifacts that may affect oxygen isotope abundances during ion probe measurements, such as the crystal orientation of chrome-spinel. We confidently classified the remnant chrome-spinels of the Jurassic using the compiled database, and compared the meteorite abundances to other time periods. However, it was difficult to determine the subgroup for a few of the ordinary-chondrite-like grains using chemistry and isotopes alone. In this event, (scanning) transmission electron microscope techniques were implemented to study the silicate inclusions within these chrome-spinels to help determine their parent meteorite type. Overall, this work supports the use of chemistry, oxygen isotopes, and inclusions to classify remnant chrome-spinels and demonstrates that meteorite populations from the past are different than today.
Geology, chrome-spinel, chromite, Jurassic, meteorites, oxygen isotopes
211 pages
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