Formation and deformation of Phosphorus-Olivine-Assemblages in the Chelyabinsk chondrite. (arXiv:2007.11137v1 [astro-ph.EP])
<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 unstrained phosphate populations. Unstrained
(recrystallized and annealed) grains 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 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 unstrained phosphate populations. Unstrained
(recrystallized and annealed) grains 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 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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