3:45 PM - 4:00 PM
[PPS08-20] Petrography of the cm-sized dark clast in NWA7678 reduced CV3 chondrite using Raman spectroscopy
Some chondrites are recognized as brecciated meteorites that originate from different parent bodies, or that have experienced various kinds of hydrothermal alteration within the same parent body. The clasts from such brecciated meteorites are important for understanding the formation process and environments of the host parent body. For example, the Kaidun meteorite contains clasts of many types of meteorites, and the mineralogy of the clasts and dating of their formation provided constraints on the formation age of the Kaidun parent body [1]. Since the CV chondrites are likely to contain dark inclusions that originate from the same parent body and have different degrees of alteration relative to those of host regions, the mineralogy and chemical compositions of these inclusions have been discussed the diversity of environments within the CV parent body [2]. Thus, it is important to estimate both the origin and formation of clasts and host meteorite with parent bodies in the early solar system.
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. In the JPGU 2023 presentation, we clarified the differences between the host and clast, providing a detailed petrographic description of the host and clast lithologies and constituent minerals. The dark crust has characteristics of constituent minerals not found in the host, such as alterations of olivine and pyroxene in chondrules to Fe-rich olivine, which shows a pseudomorph texture, and indicates the absence of sulfur-bearing minerals throughout the large dark clast (2 x 4 cm), 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. 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 different parent body of the host parts as reduced CV3 chondrites.
On the other hands, in order to give a constrain for the origin and formation of the clast, the detailed petrological characteristics, analyses of stable isotopic compositions of hydrogen, oxygen, and carbon, and radioactive isotope chronology should be applied. In this presentation, we focus on to identify minerals in the clasts, especially Ca-carbonates (e.g., calcite or aragonite), which could not be identified by SEM-EDS, and aim to identify the minerals using Raman spectroscopy prior to isotope analyses.
By the results of Raman spectroscopy, the Ca-carbonates on the clasts are identified as some aragonites (CaCO3), and the phosphates surrounding these aragonites are merrillite (Ca9NaMg(PO4)7). Aragonite has been previously founded in CM2.2-2.5, suggesting that it formed in environments with high Mg/Ca ratios and water temperatures above 13 ℃ [3]. On the other hand, another possible factor in the formation of aragonite is the phase transition from calcite under high pressure and temperature [4]. Merrillite was reported from meteorites that are thought to have undergone thermal metamorphism, such as ordinary chondrites [5], suggesting that it was formed from phosphate by thermal metamorphism on the parent body [6]. The presence of that merrillite surrounding the aragonite suggests that the aragonite may have been formed under high pressure and high temperature, related to the process by which the merrillite was formed.
In the future, we will apply the high precision U-Pb dating by LA-ICP-MS to these aragonites to discuss the origin and genesis of this dark clast based on chronological constraints.
[1] Petitat et al. (2011), MAPS, 46, 275-283. [2] Kojima and Tomeoka (1996), GCA, 60, 2651-2666. [3] Lee et al. (2014), GCA, 144, 126-156. [4] Jamieson (1953), The Journal of Chemical Physics, 21, 1385-1390. [5] Rubin (1997), MAPS, 32, 231-247. [6] Jones et al. (2014), GCA, 132, 120-140
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. In the JPGU 2023 presentation, we clarified the differences between the host and clast, providing a detailed petrographic description of the host and clast lithologies and constituent minerals. The dark crust has characteristics of constituent minerals not found in the host, such as alterations of olivine and pyroxene in chondrules to Fe-rich olivine, which shows a pseudomorph texture, and indicates the absence of sulfur-bearing minerals throughout the large dark clast (2 x 4 cm), 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. 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 different parent body of the host parts as reduced CV3 chondrites.
On the other hands, in order to give a constrain for the origin and formation of the clast, the detailed petrological characteristics, analyses of stable isotopic compositions of hydrogen, oxygen, and carbon, and radioactive isotope chronology should be applied. In this presentation, we focus on to identify minerals in the clasts, especially Ca-carbonates (e.g., calcite or aragonite), which could not be identified by SEM-EDS, and aim to identify the minerals using Raman spectroscopy prior to isotope analyses.
By the results of Raman spectroscopy, the Ca-carbonates on the clasts are identified as some aragonites (CaCO3), and the phosphates surrounding these aragonites are merrillite (Ca9NaMg(PO4)7). Aragonite has been previously founded in CM2.2-2.5, suggesting that it formed in environments with high Mg/Ca ratios and water temperatures above 13 ℃ [3]. On the other hand, another possible factor in the formation of aragonite is the phase transition from calcite under high pressure and temperature [4]. Merrillite was reported from meteorites that are thought to have undergone thermal metamorphism, such as ordinary chondrites [5], suggesting that it was formed from phosphate by thermal metamorphism on the parent body [6]. The presence of that merrillite surrounding the aragonite suggests that the aragonite may have been formed under high pressure and high temperature, related to the process by which the merrillite was formed.
In the future, we will apply the high precision U-Pb dating by LA-ICP-MS to these aragonites to discuss the origin and genesis of this dark clast based on chronological constraints.
[1] Petitat et al. (2011), MAPS, 46, 275-283. [2] Kojima and Tomeoka (1996), GCA, 60, 2651-2666. [3] Lee et al. (2014), GCA, 144, 126-156. [4] Jamieson (1953), The Journal of Chemical Physics, 21, 1385-1390. [5] Rubin (1997), MAPS, 32, 231-247. [6] Jones et al. (2014), GCA, 132, 120-140