17:15 〜 19:15
[SIT21-P03] The late thermal events of carbonaceous chondrites revealed by I-Xe dating and halogen analyses
Carbonaceous chondrites are meteorites that contain various organic compounds and abundant (de)hydrated minerals. The thermal processes on their parent bodies are important for exploring the temperature distribution and physical and chemical evolution of the protoplanetary disk. However, the timing of the thermal events has not been clarified. Halogen concentrations and noble gas isotope ratios in these meteorites are important for understanding the water-rock interaction and thermal processes on parent bodies.
Here, we determined halogen (Cl, Br, I) concentrations and I-Xe ages of small chips from several carbonaceous chondrites (Allende CV3.7, NWA801 CR2.8, Jbilet Winselwan CM2.3-2.7, Aguas Zarcas CM1-2.8, Tagish Lake C2) to gain insight into the distribution of halogens concentrations, and the timing of thermal events on their parent bodies. For halogen analysis, 3 g of Allende whole rock sample was crushed and sieved to produce four powder fractions (>150 μm) ranging from 1.4 to 17.1 mg. In addition, two small chip samples of 15.9 and 33.1 mg were used for analysis. The 33.1 mg chip was also used for I-Xe dating. For halogen analysis and I-Xe dating, additional small chips (< 3mm) were prepared for NWA 801 34.5 mg, Aguas Zarcas 30.4 mg, Jbilet Winselwan 23 mg, Tagish Lake 29.9 mg. All samples were irradiated with neutrons at the Kyoto Univ. Research Reactor. Noble gas isotope analyses (Ar, Kr, Xe) were performed using a modified VG3600 mass spectrometer at the Univ. of Tokyo. The noble gases were extracted by stepwise heating the samples between 500 and 1800 ℃. Halogen compositions were determined based on halogen-derived noble gas isotopes by neutron irradiation, 38Ar for Cl, 80Kr for Br, and 128Xe for I, respectively.
The average halogen concentrations obtained from Allende were consistent with each other regardless of the sample form (powder or chip), which were in agreement with those in some previous studies regardless of the method of analysis[1, 2]. However, the small chip sample of 33.1 mg showed significantly higher concentrations for all halogens, suggesting the existence of intra-sample heterogeneity in halogen concentrations. Based on the halogen concentration analysis of carbonaceous chondrites, the halogen release patterns were compared across the low (500-690°C), medium (720-1100°C), and high (1200-1800°C) temperature ranges. In the medium and high-temperature phases, the majority of halogens (approximately 90%) were released from Allende. NWA 801 and Jbilet Winselwan showed a moderate release of about 60%. However, Aguas Zarcas and Tagish Lake meteorites had a significantly smaller release of only about 30%, with release mainly at low-temperature ranges. This difference in release patterns likely reflects the peak temperatures reached during thermal metamorphism, and is consistent with previous studies, which reported the following peak temperatures: Allende, ~1000°C; Jbilet Winselwan, 400-500°C; Aguas Zarcas, <400°C; and Tagish Lake, ~100°C [3-6]. In addition, the 128Xe release pattern showed that Aguas Zarcas and Tagish Lake, which had mainly low-temperature releases of halogens, also had about 60% of the total 128Xe release at low temperatures (500-690°C). All I-Xe ages yielded for these meteorites showed upper limits around 4.5 Ga and the margin of error was too large to derive age values from the slope of I-Xe isochron. On the other hand, for meteorites with significant 128Xe releases at medium and high temperatures (>60%), I-Xe ages were obtained, showing 4558±4 Ma for Allende, 4491±19 Ma for NWA 801, and 4517±15 Ma for Jbilet Winselwan. The age obtained in Allende is younger than the age of aqueous alteration that occurred in the CV3 parent body, 4563-4564 Ma [7], and is consistent with the results of thermochronological modeling [8] showing that temperatures of about 400°C persisted around 4559 Ma. The age obtained in NWA801 is even younger than ~4553 Ma of the GRO 95577 [9], which showed the youngest age of aqueous alteration, and the age obtained in Jbilet Winselwan also showed a younger age than the timing of aqueous alteration 4564.7±1 Ma [10]. This suggests that the I-Xe age indicates the timing of post-aqueous alteration thermal disturbance. Our results suggest that thermal metamorphism occurred on multiple carbonaceous chondrite parent bodies several to several tens of million years or more after aqueous alteration.
Refs: [1] Ozaki and Ebihara (2007), [2] Lodders and Fegley Jr. (2023), [3] Weinbruch et al. (1994), [4] King et al. (2018), [5] Kerraouch et al. (2022), [6] Schrader et al. (2024), [7] Jogo et al. 2017 GCA, [8] Carporzen et al. (2010), [9] Jilly-Rehak et al. (2017), [10] Fujiya et al. (2022).
Here, we determined halogen (Cl, Br, I) concentrations and I-Xe ages of small chips from several carbonaceous chondrites (Allende CV3.7, NWA801 CR2.8, Jbilet Winselwan CM2.3-2.7, Aguas Zarcas CM1-2.8, Tagish Lake C2) to gain insight into the distribution of halogens concentrations, and the timing of thermal events on their parent bodies. For halogen analysis, 3 g of Allende whole rock sample was crushed and sieved to produce four powder fractions (>150 μm) ranging from 1.4 to 17.1 mg. In addition, two small chip samples of 15.9 and 33.1 mg were used for analysis. The 33.1 mg chip was also used for I-Xe dating. For halogen analysis and I-Xe dating, additional small chips (< 3mm) were prepared for NWA 801 34.5 mg, Aguas Zarcas 30.4 mg, Jbilet Winselwan 23 mg, Tagish Lake 29.9 mg. All samples were irradiated with neutrons at the Kyoto Univ. Research Reactor. Noble gas isotope analyses (Ar, Kr, Xe) were performed using a modified VG3600 mass spectrometer at the Univ. of Tokyo. The noble gases were extracted by stepwise heating the samples between 500 and 1800 ℃. Halogen compositions were determined based on halogen-derived noble gas isotopes by neutron irradiation, 38Ar for Cl, 80Kr for Br, and 128Xe for I, respectively.
The average halogen concentrations obtained from Allende were consistent with each other regardless of the sample form (powder or chip), which were in agreement with those in some previous studies regardless of the method of analysis[1, 2]. However, the small chip sample of 33.1 mg showed significantly higher concentrations for all halogens, suggesting the existence of intra-sample heterogeneity in halogen concentrations. Based on the halogen concentration analysis of carbonaceous chondrites, the halogen release patterns were compared across the low (500-690°C), medium (720-1100°C), and high (1200-1800°C) temperature ranges. In the medium and high-temperature phases, the majority of halogens (approximately 90%) were released from Allende. NWA 801 and Jbilet Winselwan showed a moderate release of about 60%. However, Aguas Zarcas and Tagish Lake meteorites had a significantly smaller release of only about 30%, with release mainly at low-temperature ranges. This difference in release patterns likely reflects the peak temperatures reached during thermal metamorphism, and is consistent with previous studies, which reported the following peak temperatures: Allende, ~1000°C; Jbilet Winselwan, 400-500°C; Aguas Zarcas, <400°C; and Tagish Lake, ~100°C [3-6]. In addition, the 128Xe release pattern showed that Aguas Zarcas and Tagish Lake, which had mainly low-temperature releases of halogens, also had about 60% of the total 128Xe release at low temperatures (500-690°C). All I-Xe ages yielded for these meteorites showed upper limits around 4.5 Ga and the margin of error was too large to derive age values from the slope of I-Xe isochron. On the other hand, for meteorites with significant 128Xe releases at medium and high temperatures (>60%), I-Xe ages were obtained, showing 4558±4 Ma for Allende, 4491±19 Ma for NWA 801, and 4517±15 Ma for Jbilet Winselwan. The age obtained in Allende is younger than the age of aqueous alteration that occurred in the CV3 parent body, 4563-4564 Ma [7], and is consistent with the results of thermochronological modeling [8] showing that temperatures of about 400°C persisted around 4559 Ma. The age obtained in NWA801 is even younger than ~4553 Ma of the GRO 95577 [9], which showed the youngest age of aqueous alteration, and the age obtained in Jbilet Winselwan also showed a younger age than the timing of aqueous alteration 4564.7±1 Ma [10]. This suggests that the I-Xe age indicates the timing of post-aqueous alteration thermal disturbance. Our results suggest that thermal metamorphism occurred on multiple carbonaceous chondrite parent bodies several to several tens of million years or more after aqueous alteration.
Refs: [1] Ozaki and Ebihara (2007), [2] Lodders and Fegley Jr. (2023), [3] Weinbruch et al. (1994), [4] King et al. (2018), [5] Kerraouch et al. (2022), [6] Schrader et al. (2024), [7] Jogo et al. 2017 GCA, [8] Carporzen et al. (2010), [9] Jilly-Rehak et al. (2017), [10] Fujiya et al. (2022).