16:00 〜 16:15
[PPS08-21] Experimental evidence of phase transition of silica polymorphs in basaltic eucrites
キーワード：シリカ鉱物、等温加熱実験、玄武岩質ユークライト
Eucrites originated from the surface crust of protoplanet in the early solar system, probably asteroid 4 Vesta [1]. Eucrites are mainly composed of pyroxene, plagioclase, and several accessory minerals include silica minerals. The silica mineral has several polymorphs under different P/T conditions [2]. The identification of silica polymorphs provides information about the thermal and shock history of the parent body. In this study, we performed isothermal heating experiments on eucrites to evaluate the thermal history of Vestan crust.
We selected three basaltic eucrites for our study: A-87272 (metamorphic type 7, shock degree E), Agoult (metamorphic type 5, shock degree A), and, HaH 262 (metamorphic type 4, shock degree B) [3-5]. We performed petrographic observations of polished sections using an FE-SEM (JEOL JSM-7100) equipped with a ChromaCL2 (GATAN), and a µ-Raman spectroscope (Renishaw InVia) at NIPR. In this study, we performed isothermal experiments (Table 1). Additionally, we used run products created in heating experiments conducted by Yamaguchi et al. [6].
Our experiments showed that silica, MC tridymite, quartz, cristobalite, and silica glass, exhibited different transition processes into other polymorphs. First, all silica minerals in unheated Agoult were identified as MC tridymite, and the MC tridymite did not recrystallize into other polymorphs during our experiments. Next, the silica minerals in unheated HaH 262 were identified as a significant amount of quartz and a minor cristobalite. The quartz did not recrystallize into other polymorphs up to 1010 ºC, but recrystallized into cristobalite heated above 1040 ºC. Finally, most of the silica in unheated A-87272 was identified as silica glass, and the silica glass recrystallized into quartz up to 1000 ºC, and recrystallized into cristobalite above 1040 ºC. Meanwhile, the solidus temperature of the eucrite is ~1060 ºC [7], and extensive partial melting (>~10 vol.%) was observed in samples heated above 1070 ºC. In these samples, crystallization of PO tridymite in contact with the melts was observed.
Ono et al. [8] suggested that the presence of silica polymorphs in eucrites is associated with cooling rates. They proposed that cristobalite initially crystallizes from eucritic magma and subsequently transitions to other polymorphs during the cooling process. Furthermore, they proposed that tridymite recrystallized into quartz as a result of secondary heating. This hypothesis is supported by their heating experiments of Y 980433 (cumulate eucrite, shock degree D). In this study, our observations did not support the recrystallization of tridymite to quartz during heating experiments. This difference may be due to the starting material. Typically, large amount of silica glass coexisting with MC tridymite are observed in Y 980433 and other eucrites above shock degree D. Additionally, the Y 980433 originally contains quartz in the sample before heating, making it challenging to distinguish this original quartz from any potentially newly formed quartz during the heating experiment. Therefore, based on our experiments, we concluded that tridymite does not recrystallize into quartz through reheating events, while silica glass is a precursor capable of recrystallization into quartz and cristobalite. We also observed the crystallization of PO tridymite from quenched melts. This fact suggests that the first silica mineral to crystallize in eucritic magma is not necessarily cristobalite. These facts require a reconsideration of the formation process of silica minerals in eucrites.
Reference
[1] Binzel and Xu (1993) Science 260, 186-191. [2] Heaney (1994) Reviews in Mineralogy, 29, 1-40. [3] Takeda and Graham (1991) Meteoritics 26, 129-134. [4] Yamaguchi et al. (1996) Icarus 124, 97-112. [5] Kanemaru et al. (2020) Polar Sci. 26, 100605. [6] Stolper (1977) Geochim. Cosmochim. Acta. 41, 5, 587-611. [7] Yamaguchi et al. (2013) [8] Ono et al. (2021) Meteoritics and Planetary Sci. 56, 1086-1108.
We selected three basaltic eucrites for our study: A-87272 (metamorphic type 7, shock degree E), Agoult (metamorphic type 5, shock degree A), and, HaH 262 (metamorphic type 4, shock degree B) [3-5]. We performed petrographic observations of polished sections using an FE-SEM (JEOL JSM-7100) equipped with a ChromaCL2 (GATAN), and a µ-Raman spectroscope (Renishaw InVia) at NIPR. In this study, we performed isothermal experiments (Table 1). Additionally, we used run products created in heating experiments conducted by Yamaguchi et al. [6].
Our experiments showed that silica, MC tridymite, quartz, cristobalite, and silica glass, exhibited different transition processes into other polymorphs. First, all silica minerals in unheated Agoult were identified as MC tridymite, and the MC tridymite did not recrystallize into other polymorphs during our experiments. Next, the silica minerals in unheated HaH 262 were identified as a significant amount of quartz and a minor cristobalite. The quartz did not recrystallize into other polymorphs up to 1010 ºC, but recrystallized into cristobalite heated above 1040 ºC. Finally, most of the silica in unheated A-87272 was identified as silica glass, and the silica glass recrystallized into quartz up to 1000 ºC, and recrystallized into cristobalite above 1040 ºC. Meanwhile, the solidus temperature of the eucrite is ~1060 ºC [7], and extensive partial melting (>~10 vol.%) was observed in samples heated above 1070 ºC. In these samples, crystallization of PO tridymite in contact with the melts was observed.
Ono et al. [8] suggested that the presence of silica polymorphs in eucrites is associated with cooling rates. They proposed that cristobalite initially crystallizes from eucritic magma and subsequently transitions to other polymorphs during the cooling process. Furthermore, they proposed that tridymite recrystallized into quartz as a result of secondary heating. This hypothesis is supported by their heating experiments of Y 980433 (cumulate eucrite, shock degree D). In this study, our observations did not support the recrystallization of tridymite to quartz during heating experiments. This difference may be due to the starting material. Typically, large amount of silica glass coexisting with MC tridymite are observed in Y 980433 and other eucrites above shock degree D. Additionally, the Y 980433 originally contains quartz in the sample before heating, making it challenging to distinguish this original quartz from any potentially newly formed quartz during the heating experiment. Therefore, based on our experiments, we concluded that tridymite does not recrystallize into quartz through reheating events, while silica glass is a precursor capable of recrystallization into quartz and cristobalite. We also observed the crystallization of PO tridymite from quenched melts. This fact suggests that the first silica mineral to crystallize in eucritic magma is not necessarily cristobalite. These facts require a reconsideration of the formation process of silica minerals in eucrites.
Reference
[1] Binzel and Xu (1993) Science 260, 186-191. [2] Heaney (1994) Reviews in Mineralogy, 29, 1-40. [3] Takeda and Graham (1991) Meteoritics 26, 129-134. [4] Yamaguchi et al. (1996) Icarus 124, 97-112. [5] Kanemaru et al. (2020) Polar Sci. 26, 100605. [6] Stolper (1977) Geochim. Cosmochim. Acta. 41, 5, 587-611. [7] Yamaguchi et al. (2013) [8] Ono et al. (2021) Meteoritics and Planetary Sci. 56, 1086-1108.