13:45 〜 14:00
[PPS07-13] Silica Minerals as Recorders of Crystallization, Thermal, and Shock Histories of Eucrites: Insights into Vesta’s Crustal Evolution
★Invited Papers
キーワード：シリカ鉱物、カソードルミッセンス、ユークライト隕石
Silica minerals have been identified as accessory minerals in some meteorite groups [e.g., 1]. These minerals are known to have various polymorphs under different P/T conditions. Understanding these minerals is essential for revealing the thermal and shock history of rocks. This study investigates silica polymorphs in fourteen eucrites, thought to originate from the crust of 4 Vesta. Silica minerals in eucrites likely crystallized from residual melts during the last stages of magma solidification [2].
We used a laser Raman spectroscopy (Renishaw), and a FE-SEM (JEOL JSM-7100) equipped with an EDS (Oxford AZtec Energy) and a CL imager (GATAN) at NIPR. The Raman and SEM-CL combined method distinguishes silica polymorphs by detecting variations in CL emission colors and acquiring Raman spectra from areas showing different color tones, allowing for the two-dimensional mapping of silica polymorphs. The chemical compositions of each silica polymorph were obtained using an EPMA (JEOL JXA-8200) at NIPR.
Silica minerals in eucrites show varying proportions, allowing classification into cristobalite-dominant, quartz-dominant, quartz-tridymite-dominant, and tridymite-dominant eucrites. Tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexisting with opaques and phosphates (i.e., mesostaisis). These occurrences suggest that silica polymorphs crystallize through different stages and processes. Minor element analysis revealed a trend where alkali components (Na2O, K2O) and Al2O3 increase in the order of quartz, cristobalite, and tridymite. This suggests that tridymite crystallized from a melt enriched in alkalis and Al2O3. Powell et al. [3] proposed that immiscible melts (rich in Si, K, and Al) can form in eucrite magma under slow cooling, making such melts the most likely candidate for euhedral tridymite. In contrast, experiments have shown that euhedral cristobalite crystallizes from rapidly cooled eucrite melts [1]. The cristobalite observed in eucrites shows textures resembling experimental products, suggesting it crystallized through rapid cooling from the solidus temperature (1060ºC). Unlike tridymite or cristobalite, quartz in eucrites is characterized by a low TiO2 content (below 0.04 wt.%). Given the coexistence of quartz with abundant ilmenite, the application of the TitaniQ geothermometer [4] suggests a crystallization temperature below 1000ºC. In alkali-depleted systems, the stability fields of quartz and cristobalite are divided at approximately 1025ºC [5]. Therefore, the quartz in eucrites is suggested to have crystallized at lower temperature compared to other silica minerals. Notably, quartz forms mesostasis regions along with phosphate and opaque minerals. Such partial melts, enriched in volatile and incompatible elements, locally contribute to lowering the solidus temperature of the eucrite.
Furthermore, silica glass is commonly observed in eucrites with a shock degrees D and E [6]. These silica glasses coexist with tridymite and exhibit a minor element composition similar to that of the tridymite. These facts suggest that tridymite converted into diaplectic glass due to shock events. Recently, Kanemaru et al. [7] reported that these silica glasses recrystallize into quartz (heated at 900-1010 ºC for 100 h) cristobalite due to secondary heating events. Hence, the silica minerals reflect a history of secondary shock and thermal metamorphism after crystallizing from magma, containing crucial information about the planetary formation history of the early solar system.
[1] Ono et al. Meteorit. Planet. Sci. 56, 1086–1108 (2021). [2] Stolper. E., Geochim. Cosmochim. Acta. 41, 587-611 (1977). [3] Powel M.A., et al. Proc. Planet. Sci. Conf. 2, 1153–1168 (1980). [4] Wark D. A. and Watson E. D., Contrib Mineral Petrol 152, 743–754 (2006). [5] Holmquist S. B., J. Am. Ceram. Soc. 44, 82–86 (1961). [6] Kanemaru et al. Pol. Sci. 26, 100605 (2020). [7] Kanemaru et al. Sci. Rep. 14, 26414 (2024)
We used a laser Raman spectroscopy (Renishaw), and a FE-SEM (JEOL JSM-7100) equipped with an EDS (Oxford AZtec Energy) and a CL imager (GATAN) at NIPR. The Raman and SEM-CL combined method distinguishes silica polymorphs by detecting variations in CL emission colors and acquiring Raman spectra from areas showing different color tones, allowing for the two-dimensional mapping of silica polymorphs. The chemical compositions of each silica polymorph were obtained using an EPMA (JEOL JXA-8200) at NIPR.
Silica minerals in eucrites show varying proportions, allowing classification into cristobalite-dominant, quartz-dominant, quartz-tridymite-dominant, and tridymite-dominant eucrites. Tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexisting with opaques and phosphates (i.e., mesostaisis). These occurrences suggest that silica polymorphs crystallize through different stages and processes. Minor element analysis revealed a trend where alkali components (Na2O, K2O) and Al2O3 increase in the order of quartz, cristobalite, and tridymite. This suggests that tridymite crystallized from a melt enriched in alkalis and Al2O3. Powell et al. [3] proposed that immiscible melts (rich in Si, K, and Al) can form in eucrite magma under slow cooling, making such melts the most likely candidate for euhedral tridymite. In contrast, experiments have shown that euhedral cristobalite crystallizes from rapidly cooled eucrite melts [1]. The cristobalite observed in eucrites shows textures resembling experimental products, suggesting it crystallized through rapid cooling from the solidus temperature (1060ºC). Unlike tridymite or cristobalite, quartz in eucrites is characterized by a low TiO2 content (below 0.04 wt.%). Given the coexistence of quartz with abundant ilmenite, the application of the TitaniQ geothermometer [4] suggests a crystallization temperature below 1000ºC. In alkali-depleted systems, the stability fields of quartz and cristobalite are divided at approximately 1025ºC [5]. Therefore, the quartz in eucrites is suggested to have crystallized at lower temperature compared to other silica minerals. Notably, quartz forms mesostasis regions along with phosphate and opaque minerals. Such partial melts, enriched in volatile and incompatible elements, locally contribute to lowering the solidus temperature of the eucrite.
Furthermore, silica glass is commonly observed in eucrites with a shock degrees D and E [6]. These silica glasses coexist with tridymite and exhibit a minor element composition similar to that of the tridymite. These facts suggest that tridymite converted into diaplectic glass due to shock events. Recently, Kanemaru et al. [7] reported that these silica glasses recrystallize into quartz (heated at 900-1010 ºC for 100 h) cristobalite due to secondary heating events. Hence, the silica minerals reflect a history of secondary shock and thermal metamorphism after crystallizing from magma, containing crucial information about the planetary formation history of the early solar system.
[1] Ono et al. Meteorit. Planet. Sci. 56, 1086–1108 (2021). [2] Stolper. E., Geochim. Cosmochim. Acta. 41, 587-611 (1977). [3] Powel M.A., et al. Proc. Planet. Sci. Conf. 2, 1153–1168 (1980). [4] Wark D. A. and Watson E. D., Contrib Mineral Petrol 152, 743–754 (2006). [5] Holmquist S. B., J. Am. Ceram. Soc. 44, 82–86 (1961). [6] Kanemaru et al. Pol. Sci. 26, 100605 (2020). [7] Kanemaru et al. Sci. Rep. 14, 26414 (2024)