[P-PS25_2AM2] 隕石解剖学: 太陽系物質の総合的理解に向けて

[PPS25-13] 電子線ホログラフィーによるフランボイダルマグネタイトの磁場構造と生成過程の解明

*木村 勇気¹、佐藤 岳志²、中村 智樹¹、中村 教博¹、野澤 純³、塚本 勝男¹、山本 和生⁴ (1.東北大学大学院理学研究科地学専攻、2.（株）日立ハイテクノロジーズ、3.東北大学金属材料研究所、4.ファインセラミックスセンター)

キーワード：マグネタイト, 電子線ホログラフィー, タギシュレイク隕石

Small solar system bodies were formed as agglomerates of dust and ices 4.6 billion years ago. Several million years after asteroid formation [1], the ice melted due to radioactive heating inside the larger asteroids [2] and/or highly energetic impacts [3]. Then, water plays several major roles in the chemistry of asteroids, both in mineralization and in the formation of organic compounds. Currently, bulk liquid water no longer exists in meteorites. We see only the signature of water in ancient asteroids as veins of hydrothermally deposited minerals [4] or water trapped in salt crystals [5] in meteorites. The Tagish Lake meteorite, which is a unique Type 2 carbonaceous chondrite, has a signature of aqueous process in the matrix that is abundant micrometer-sized polyhedral particles of magnetite [6]. The framboids are three-dimensionally ordered colloidal crystals of magnetite nanoparticles. The uniformity of the size distribution and the similar morphology of the magnetite nanoparticles in each of the colloidal crystals suggest that they were formed through homogeneous nucleation from a highly supersaturated isolated solution in a single nucleation event. Here we show evidence of how magnetite nanoparticles assembled into periodic structures based on a nanometer scale paleomagnetic method using electron holography in an examination of the framboidal magnetite in the Tagish Lake carbonaceous chondrite [7]. An attractive force such as magnetism never contributes to the formation of colloidal crystals [8], but the repulsive force caused by the surface charge of the magnetite is able to work. To overcome the repulsive force, the density of magnetite nanoparticles in a solution must be sufficiently high in an isolated solution as a water droplet parches in microgravity. We used electron holography to visualize the magnetization of the meteoritic minerals for the first time and found that magnetite grains in the framboid have no external magnetic force, i.e., they have a flux-closure vortex structure, which allowed the formation and preservation of the colloidal crystals. We conclude that these framboids formed in tiny water droplets with pH of 7-12 containing ions such as Ca²⁺ and Mg²⁺ at levels of 10^-14-10^-15 mol m^-2, just before exhaustion of water during thermal alteration in a hydrous asteroid.[1] Fujiya, W., Sugiura, N., Hotta, H., Ichimura, K. & Sano, Y. Evidence for the late formation of haydrous asteroids from young meteoritic carbonates. Nature Communications 3, 627 (2012).[2] Endress, M., Zinner, E. & Bischoff, A. Early aqueous activity on primitive meteorite parent bodies. Nature 379, 701-703 (1996).[3] Rubin, A. F. Collisional facilitation of aqueous alteration of CM and CV carbonaceous chondrites. Geochim. Cosmochim. Acta 90, 181-194 (2012).[4] Tomeoka, K. Phyllosilicate veins in a CI meteorite: evidence for aqueous alteration on the parent body. Nature 345, 138-140 (1990).[5] Zolensky, M. E. et al. Asteroidal water within fluid inclusion-bearing halite in an H5 chondrite, Monahans (1998). Science 285, 1377-1379 (1999).[6] Nozawa, J. et al. Magnetite 3-D Colloidal Crystals Formed in the Early Solar System 4.6 Billion Years Ago, Journal of the American Chemical Society, 133, 8782-8785(2011).[7] Kimura, Y. et al. Vortex magnetic structure in framboidal magnetite reveals existence of water droplets in an ancient asteroid, Nature Communications, 4 (2013) 2649 doi: 10.1038/ncomms3649.[8] Philipse, A. P. & Maas, D. Magnetic colloids from magnetotactic bacteria: chain formation and colloidal stability. Langmuir 18, 9977-9984 (2002).