11:45 〜 12:00
[PPS09-11] Cracking the Martian Code: Olivine Fractures, Clay Clues, and Aqueous Whispers in Nakhlite Meteorites
キーワード：Nakhlites, Aqueous alteration, Iddingsite, Olivine fractures, saponite
Martian clays (phyllosilicates), owing to their potential for preservation of biosignatures, are essential in shedding light pertaining to the planet’s paleoclimatic condition and, hence, have been a target for recent Martian missions. Thus, a comprehensive study was performed on the textural and compositional characteristics of the secondary phases in Nakhlite meteorites Yamato-593, Miller-003346, Lafayette, and Nakhla. The study aimed to uncover how different physical and chemical factors influenced the aqueous alteration of olivine under Martian conditions, leading to the formation of clay minerals.
The meteorites, which have a clinopyroxene-rich cumulate structure, are strongly fractured. Under petrographic examination, brownish clay accumulations were generally found along fractures in olivine grains. The main objective of this research was to investigate the effects of aqueous alteration on olivine and to establish the geochemical conditions required for clay deposition. Raman spectroscopy of the altered materials along these fractures showed that the deposits are mainly composed of saponite and iron oxides, which are heterogeneously mixed and collectively referred to as iddingsite. The confinement of clay deposits within olivine fractures suggests they formed in conditions with low water-to-rock ratios. When comparing meteorite samples, it was found that Yamato-593 (Y-593), Lafayette (Laf), and Nakhla (Nak) all contained olivine with a consistent fayalitic (Fa) composition (Fa68-72), while Miller-00346 (MIL-3346) had more forsterite-rich olivine (Fa59). In all the samples, saponite was the main secondary clay mineral, forming mostly along olivine fractures and often found alongside iron oxides. This combination of minerals, known as iddingsite, provides clues about the processes that led to aqueous alteration on the Martian crust.
BSE images showed that the olivine grains had serrated fractures filled with iddingsite. In Y-593, MIL-3346, and Nakhla, the uniform alteration suggests a single-phase event, while the variations in Lafayette point to multiple water pulses over time. The study focused on understanding how the Nakhlites were altered and the characteristics of their fractures. The consistent patterns in Y-593, MIL-3346, and Nakhla stood out compared to Lafayette’s unique behavior, hinting at different alteration histories. X-ray elemental maps supported this by showing cation movement from nearby minerals into the altered regions. Interestingly, Lafayette had excess aluminum, surpassing the levels in nearby plagioclase, which may indicate alteration under higher water-to-rock ratios or influence from aluminum-rich external fluids. Geochemical analysis, using bi-variant plots, showed silica enrichment in the altered products, pointing to an acidic environment. However, in Lafayette, the discovery of carbonate, along with phyllosilicates, suggested a shift from acidic to alkaline conditions. These findings, based on detailed analysis, help enhance our understanding of Martian geology and offer more evidence of the planet’s potential history of liquid water.
The meteorites, which have a clinopyroxene-rich cumulate structure, are strongly fractured. Under petrographic examination, brownish clay accumulations were generally found along fractures in olivine grains. The main objective of this research was to investigate the effects of aqueous alteration on olivine and to establish the geochemical conditions required for clay deposition. Raman spectroscopy of the altered materials along these fractures showed that the deposits are mainly composed of saponite and iron oxides, which are heterogeneously mixed and collectively referred to as iddingsite. The confinement of clay deposits within olivine fractures suggests they formed in conditions with low water-to-rock ratios. When comparing meteorite samples, it was found that Yamato-593 (Y-593), Lafayette (Laf), and Nakhla (Nak) all contained olivine with a consistent fayalitic (Fa) composition (Fa68-72), while Miller-00346 (MIL-3346) had more forsterite-rich olivine (Fa59). In all the samples, saponite was the main secondary clay mineral, forming mostly along olivine fractures and often found alongside iron oxides. This combination of minerals, known as iddingsite, provides clues about the processes that led to aqueous alteration on the Martian crust.
BSE images showed that the olivine grains had serrated fractures filled with iddingsite. In Y-593, MIL-3346, and Nakhla, the uniform alteration suggests a single-phase event, while the variations in Lafayette point to multiple water pulses over time. The study focused on understanding how the Nakhlites were altered and the characteristics of their fractures. The consistent patterns in Y-593, MIL-3346, and Nakhla stood out compared to Lafayette’s unique behavior, hinting at different alteration histories. X-ray elemental maps supported this by showing cation movement from nearby minerals into the altered regions. Interestingly, Lafayette had excess aluminum, surpassing the levels in nearby plagioclase, which may indicate alteration under higher water-to-rock ratios or influence from aluminum-rich external fluids. Geochemical analysis, using bi-variant plots, showed silica enrichment in the altered products, pointing to an acidic environment. However, in Lafayette, the discovery of carbonate, along with phyllosilicates, suggested a shift from acidic to alkaline conditions. These findings, based on detailed analysis, help enhance our understanding of Martian geology and offer more evidence of the planet’s potential history of liquid water.