Microtextures in the Chelyabinsk impact breccia reveal the history of Phosphorus-Olivine-Assemblages in chondrites. (arXiv:2007.11137v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Walton_C/0/1/0/all/0/1">Craig R. Walton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Baziotis_I/0/1/0/all/0/1">Ioannis Baziotis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cernok_A/0/1/0/all/0/1">Ana Černok</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shorttle_O/0/1/0/all/0/1">Oliver Shorttle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Asimow_P/0/1/0/all/0/1">Paul D. Asimow</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ferriere_L/0/1/0/all/0/1">Ludovic Ferrière</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anand_M/0/1/0/all/0/1">Mahesh Anand</a>
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.
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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