[M-IS19] 水惑星学

[MIS19-10] Near-mid infrared spectra of carbonaceous chondrites: toward understanding the aqueous alteration and thermal history of volatile-rich asteroids

*Jinfei Yu^1,2,3、Hiroyuki Kurokawa¹、Cloutis A. Edward⁴、Yunzhao Wu³、Haibin Zhao³ (1.The University of Tokyo、2.The University of Science and Technology of China、3.Purple Mountain Observatory, CAS、4.University of Winnipeg)

キーワード：Aqueous alteration, IR spectroscopy, Carbonaceous Chondrites, Asteroids, Thermal metamorphism, Mineralogy

Aqueous alteration is possibly the most widespread process that has affected primitive carbonaceous chondrites (CCs) and their parent bodies (1). A large amount of secondary material was formed through water-rock reactions on planetesimals, significantly influencing the transport of water and organics to the terrestrial planet forming region in the early Solar System (2). While the infrared spectroscopy remains a powerful tool for investigating aqueous alteration, challenges persist in utilizing near-mid infrared reflectance spectral to quantify degree of aqueous alteration and explore the evolution history of volatile-rich asteroids, such as the post hydration metamorphic overprint (3). In order to determine the controls on the reflectance spectra of hydrated CCs, we investigated the 1–25 μm infrared spectral features of 17 CCs with different petrologic properties (4). We elucidated the spectral variation patterns related to aqueous alteration and thermal metamorphism across four key spectral features. As aqueous alteration progress, hydrated minerals in meteorites increase and undergo conversion to Mg-rich phases (5). Both the 3 μm band, diagnostic of OH-bearing minerals and water, and the 6 μm band, diagnostic of water molecules, both features increase in strength and band centers shift to the short-wavelength. When comparing the CCs to main-belt asteroids from AKARI’s observations (6), distribution pattern of the 3 μm band in C-complex asteroids significantly differs from the spectral variations associated with alteration and metamorphism in CCs, the general lack of asteroids matched to highly altered meteorites may indicate that CI-like carbonaceous chondrites parent bodies are rare or underwent more intense thermal metamorphism. Vibrational features of silicates control the 10μm and 20μm regions (7). The 12.4 μm to 11.4 μm reflectance ratio reduces and the reflectance peak in 9–14 μm region shifts to short-wavelength because of the conversion of anhydrous silicates to phyllosilicates. In the 15–25 μm region, with the thermal metamorphism processes, the single reflectance peak at 22.1 μm changes to two peaks at 19 μm and 25 μm, spectral curve becomes rough, which is likewise explained by the increase of anhydrous silicate with the increase of recrystallized olivine. These results can apply to future mid-infrared observation and provide new constraints on C-complex asteroids’ volatile-rich compositions and their parent bodies’ thermal evolutionary history.
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