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A record of low-temperature asteroidal processes of amoeboid olivine aggregates from the Kainsaz CO3.2 chondrite

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Title
A record of low-temperature asteroidal processes of amoeboid olivine aggregates from the Kainsaz CO3.2 chondrite
Other Titles
케인사즈 운석에서 산출되는 AOA가 기록하고 있는 소행성의 저온 프로세스
Authors
한장미
Park, Changkun
Adrian Brearley
Keywords
Amoeboid olivine aggregatesCO3 chondritesfluidthermal metamorphismtransmission electron microscopy
Issue Date
2022
Citation
한장미, Park, Changkun, Adrian Brearley. 2022. "A record of low-temperature asteroidal processes of amoeboid olivine aggregates from the Kainsaz CO3.2 chondrite". GEOCHIMICA ET COSMOCHIMICA ACTA, 322(1): 109-128.
Abstract
Amoeboid olivine aggregates (AOAs) from the Kainsaz CO3.2 chondrite were analyzed using transmission electron microscopy in order to gain a more complete understanding of thermal metamorphism on the parent body and the role of fluids during metamorphic heating. The Kainsaz AOAs are dominated by strongly zoned, fine-grained, olivine grains (Fa 2-31 ) with heterogeneous Fe enrichments along the grain boundaries, which are interpreted as the result of Fe 2+ -Mg 2+ interdiffusion with the matrix during thermal metamorphism. However, our diffusion calculations show that such AOA olivine zoning and compositions cannot be produced by a simple diffusional exchange during metamorphic heating, unlike chondrule olivine zoning and compositions. In addition, fine-grained ferroan olivine overgrowths occur heterogeneously in crystallographic continuity with olivines on the AOA exteriors. The overgrowths (Fa 33-36 ) are compositionally distinct from the underlying AOA olivines and are not fully equilibrated with the matrix olivines (Fa ~20-55 ). The ferroan olivine overgrowths likely formed by precipitation from fluids in an epitaxial relationship with forsteritic olivine on the edges of AOAs. Texturally and compositionally diverse chromite grains are also observed along olivine grain boundaries, in olivine grains, and in pore spaces between olivine grains. They share a similar crystallographic orientation relationships with adjacent olivine, suggestive of their formation by exsolution and/or epitaxial growth. Collectively, these observations provide evidence for the mobilization of Fe, Mg, Si, Cr, and Al in the presence of fluids along olivine grain boundaries and into olivine grains during thermal metamorphism. We conclude that in Kainsaz AOAs, the strong zonation development in individual olivine grains due to Fe 2+ -Mg 2+ interdiffusion and the formation of ferroan olivine overgrowths and chromite grains were a fluid-driven process that occurred at relatively low temperatures (<500℃), during the cooling history of the CO3 chondrite parent body, following the peak of thermal metamorphism.
URI
https://repository.kopri.re.kr/handle/201206/13434
DOI
http://dx.doi.org/10.1016/j.gca.2022.02.007
Type
Article
Station
기타(워크스테이션)
Indexed
SCIE
Appears in Collections  
2022-2022, Mantle dynamics and tectonic evolutions of Zealandia-Antarctic domain (22-22) / Park, Sung Hyun (PE22050)
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