17:15 〜 18:30
[MIS14-P04] 地下酸化還元勾配に沿った蛇紋岩化したオマーン・オフィオライト試料の鉱物学的・赤外分 光的特徴：火星地下の酸化還元勾配の特定に向けて
キーワード：蛇紋岩、火星、酸化還元勾配
Mars would have possessed liquid water on the surface, at least temporally, in late Noachian to early Hesperian in a dense CO2 atmosphere. In addition to surface water, groundwater is highly likely to have been abundant and interacted with mafic rocks that consisted of Martian crust. Hydrological cycles between these endmember waters would have generated redox and pH gradients in the subsurface, through vertical mixing of acidic, oxidizing surface water with alkaline, reducing groundwater. On Earth, chemoautotrophic life can live in the redox and pH gradients generated in subsurface. Thus, identifications of redox states of (sub)surface rocks on Mars through remote sensing is critical to select landing sites and to collect promising samples for microbial life in future missions to Mars. In this study, we aim to characterize mineralogical compositions and infrared spectra of serpentinized Semail Ophiolite, Oman, along with redox and pH gradients in the subsurface. Semail Ophiolite consists of basalt, gabbro, and upper mantle ophiolite, which are exposed on the surface. Oxidizing surface water infiltrates into the subsurface where in-situ serpentinization proceeds, generating steep redox and pH gradients. This geohydrological setting allows this area as a promising terrestrial analog of aqueous alterations in particular areas on Mars where ultramafic, olivine-rich rocks are exposed (e.g., Nili Fossae). We used core samples (Hole BA1B) from an area of active serpentinization of the Semail Ophiolite, drilled during the ICDP Expedition 5057 and stored at Hiroshima Univ. SEM-EPMA, XRD, and FTIR analyses were performed for the core samples. Based on mineralogical and chemical compositions of the samples, we classified core samples based on alteration stages. In the stage 1 (160–350 m in depth), samples contain unaltered minerals, e.g., olivine and diopside, and, thus, serpentinization was proceeding. The samples also contain secondary minerals of serpentine and brucite. Water chemistry of pore water at these depth were measured on site previously and reported as alkaline and reducing. The samples in the stage 1 exhibit characteristic steep slopes at 0.8–1.5 μm in infrared reflectance spectra. In the stage 2 (30–100 m in depth), serpentinization has completed without unaltered olivine or diopside. The samples are predominantly composed of serpentine and brucite. According to previous study, water chemistry of pore water was reported as alkaline and moderately oxidizing. The infrared spectra of the stage-2 samples have moderate slopes at 0.8–1.5 μm. In the stage 3 (10–30 m in depth), the samples are composed mainly of serpentine with trace Mg/Fe smectite. Fe(III) oxides appear in veins in the samples. These suggest alterations with high water-to-rock ratio with oxidizing surface water. Pore water was reported as highly oxidizing. The infrared spectra of the stage-3 samples have moderate slopes at 0.8–1.5 μm with absorption features at 1.4 and 1.9 μm owing to smectite. In the stage 4 (< 10 m in depth), the samples contain serpentine together with Mg/Fe smectite, Fe(III) oxides, and carbonate. Owing to the secondary minerals other than serpentine, the color of the samples was whitish red. The infrared spectra of the stage-4 samples have gentle slopes at 0.8–1.5 μm with clear absorption features at 1.4 and 1.9 μm. Our results show that we can distinguish the alteration stages of core samples solely based on infrared features (namely, slope gradient at 0.8–1.5 μm and absorptions at 1.4 and 1.9 μm). According to a previous microbiological study at the Semail Ophiolite site, methanogens and acetogens were found in depth of the stage 1 (and 2). Reflectance spectra features resembling to those of the stage 1 can be found in Nili Fossae regions on Mars, suggesting the importance of this area as a future landing site.