10:45 AM - 12:15 PM
[PPS08-P13] Petrography of the cm-sized dark clast in NWA7678 reduced CV3 chondrite
Chondrites have been believed to preserve the origin and evolution in the early solar system because they have not undergone differentiation since their parent body accumulation. Some chondrites are breccias that originate from different parent bodies or have undergone different alteration or metamorphism within the same parent body. The accumulation of different rock fragments in one parent body indicates that different types of meteorite parent bodies coexisted in one area of the asteroid belt at that time. It is also considered that small bodies formed in different environments crushed and accumulated repeatedly, finally accumulating in one parent body. Therefore, comparison of the petrological characteristics of the host and clast of chondrite breccias may provide constraints on their origin and genesis.
The thick section of NWA7678 reduced CV3 chondrite, which is used in this study, includes a large dark clast of 2 cm x 4 cm in size. The origin of the dark clast is not well understood because detailed petrographic study of the dark clast and comparison with the petrologic characteristics of the host have not been conducted. In this study, we aimed to constrain the origin of the dark clast by comparing the petrologic characteristics of the dark clast with the host. The differences between the host and clast have been clarified, providing a detailed petrographic description of the host and clast lithologies and constituent minerals by using SEM-EDS and FE-SEM-EDS, and comparing them. Furthermore, we provide consideration to the alteration and metamorphic history of the crust.
The results show that the chondrule, matrix abundances and average chondrule diameters in the host and dark clast differ by 61.3 %, 36.4 %, and 0.74±0.49 mm, respectively, and by 27.6 %, 70.2 %, and 0.39±0.30 mm, respectively. These petrologic characteristics of the host are consistent with those of typical CV3 chondrite (Friedrich et al., 2014). In contrast, the petrological characteristics of the dark clast are not consistent with those of chondrites of any chemical group. The dark crust also has characteristics of constituent minerals not found in the host, such as alterations of olivine and pyroxene in chondrules to Fe-rich olivine, which is considered to be a pseudomorph; the absence of sulfur-bearing minerals throughout the dark clast; the presence of Ca-carbonate coexisting with Ca and Fe-rich minerals in matrix; and the presence of unique Ca-carbonate and phosphate-rich components. In particular, pseudomorphes of olivine and unique Ca-carbonate and phosphate-rich components have not been reported in any known chondrites.
These results indicate that the dark clast have petrologic characteristics that are clearly different from those of the host and that the clast originated from a parent body different from the host. Further studies on the bulk chemical and isotopic compositions of the dark clast are needed to reveal its origin and genesis.
The thick section of NWA7678 reduced CV3 chondrite, which is used in this study, includes a large dark clast of 2 cm x 4 cm in size. The origin of the dark clast is not well understood because detailed petrographic study of the dark clast and comparison with the petrologic characteristics of the host have not been conducted. In this study, we aimed to constrain the origin of the dark clast by comparing the petrologic characteristics of the dark clast with the host. The differences between the host and clast have been clarified, providing a detailed petrographic description of the host and clast lithologies and constituent minerals by using SEM-EDS and FE-SEM-EDS, and comparing them. Furthermore, we provide consideration to the alteration and metamorphic history of the crust.
The results show that the chondrule, matrix abundances and average chondrule diameters in the host and dark clast differ by 61.3 %, 36.4 %, and 0.74±0.49 mm, respectively, and by 27.6 %, 70.2 %, and 0.39±0.30 mm, respectively. These petrologic characteristics of the host are consistent with those of typical CV3 chondrite (Friedrich et al., 2014). In contrast, the petrological characteristics of the dark clast are not consistent with those of chondrites of any chemical group. The dark crust also has characteristics of constituent minerals not found in the host, such as alterations of olivine and pyroxene in chondrules to Fe-rich olivine, which is considered to be a pseudomorph; the absence of sulfur-bearing minerals throughout the dark clast; the presence of Ca-carbonate coexisting with Ca and Fe-rich minerals in matrix; and the presence of unique Ca-carbonate and phosphate-rich components. In particular, pseudomorphes of olivine and unique Ca-carbonate and phosphate-rich components have not been reported in any known chondrites.
These results indicate that the dark clast have petrologic characteristics that are clearly different from those of the host and that the clast originated from a parent body different from the host. Further studies on the bulk chemical and isotopic compositions of the dark clast are needed to reveal its origin and genesis.