Kavli Affiliate: Paul D. Asimow
| First 5 Authors: Craig R. Walton, Ioannis Baziotis, Ana Černok, Oliver Shorttle, Paul D. Asimow
| Summary:
The geochemistry and textural associations of chondritic phosphate minerals
can provide insights into the geological histories of parental asteroids, but
the processes governing their formation and deformation remain poorly
constrained. Here, we present a quantitative assessment of phosphorus-bearing
mineral textures in the three variously-shocked lithologies (light, dark, and
melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron
microscope, electron microprobe, cathodoluminescence, and electron backscatter
diffraction techniques. Phase associations, microtextures, and microstructures
of phosphates are extremely variable within and between the differently-shocked
lithologies investigated in the Chelyabinsk meteorite. We observe continuously
strained as well as recrystallized, strain-free phosphate populations.
Recrystallized grains (with strain-free subdomains) are present only in the
more intensely shocked dark lithology, indicating that phosphate growth in
Chelyabinsk predates the development of primary shock-metamorphic textures.
This disruption event is also recorded by complete melting of portions of the
meteorite to produce the shock-melt lithology, which contains a population of
phosphorus-rich olivine grains. We interpret the textures and phase
associations of Chelyabinsk to have resulted from initial phosphate growth via
metasomatic olivine replacement, followed by major deformation during an early
shock-melting impact and a subsequent minor shock event. This minor event
appears to have generated a sub-population of phosphates that display patchy CL
textures, in both the light and dark lithology. Finally, we propose a new
classification scheme to describe various types of
Phosphorus-Olivine-Assemblages (Type I-III POAs), which can be used to classify
shock metamorphic events and define the associated physicochemical processes.
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