Date: 16:00 – 18:00, Friday, July, 8, 2022
発表者：飯塚理子
Speaker : Riko Iizuka-Oku
Title : Searching for the incorporation of noble gases into lower mantle minerals
Noble gases are important geochemical tracers for exploring global volatile cycles in the Earth’s reservoirs and evolution of Earth’s atmosphere [e.g., Marty, 2012]. In particular, Xe abundance in the atmospheres of Earth and Mars, comparing to chondrite meteorites, is more depleted than the abundance of other noble gas elements (He, Ne, Ar and Kr). The missing xenon is believed to be captured somewhere inside the Earth and various possible mechanisms. There were several experimental and computational studies on various materials, such as SiO2 polymorphs, ices, clathrates, zeolite, ferropericlase (Fp), bridgmanite (Bdg) and Fe(Ni) under crustal – mantle – core conditions, resulting in no obvious evidence for the Xe reservoir. Based on the recent lattice strain modeling, the solubility of Ar and Kr in the lower mantle are much higher than that for Xe, but still on the order of ppm due to the formation of oxygen vacancies in Fp and Bdg. Although the noble gas solubilities in Fp are systematically higher than those in Bdg, it is not easy to quantify such a tiny amount of solubilities. The two major phases in the lower mantle are unlikely to account for the missing noble gas storage.
This study aims to investigate the possibility of noble gas storage and compensation of the missing xenon in the lower mantle condition. I focused on alkali feldspars in the KAlSi3O8–NaAlSi3O8 system, the most abundant minerals in the continental crusts. KAlSi3O8 feldspar dissociates into wadeite + kyanite + coesite at 6–7 GPa and 1500 K. These three phases further recombine into KAlSi3O8 hollandite-I phase (liebermannite; tetragonal, I4/m) at ~9 GPa, 1500 K, which undergoes a phase transition into the hollandite-II phase (monoclinic, I2/m) at ~20 GPa [e.g., Sueda et al., 2004; Nishiyama et al., 2005; Ferroir et al., 2006]. Hollandite-II has a wide stability region up to 130–150 GPa, >3000 K [Hirao et al., 2008; Kawai and Tsuchiya, 2013] and is thought to be a potential host mineral transporting K and incompatible lithophile elements (LILEs) with large ionic radius into the lower mantle through subduction. Considering the crystal structure with large square tunnels accommodating large cations K+ (with a similar size to Ar) formed by four double chains of edge-shared octahedra, hollandite-II is expected to take a different and effective mechanism for noble gas incorporation from that of Fp and Bdg.
As preliminary experiments, in-situ Raman spectroscopy observations on natural feldspars were carried out using laser-hated diamond anvil cells up to 17 GPa, ~2500 K in the stability region of hollandite-I. Argon or sample in itself was used as a pressure medium. Heated spots of the recovered samples showed Raman spectra assignable to hollandite-I phase and no spectral changes due to the argon gas incorporation. Unheated areas completely turned back to initial samples. In the talk, I would also introduce research environments in Mineral Physics Laboratory at Harvard University and future plans for xenon experiments.
発表者：赤荻正樹
Speaker : Masaki Akaogi
タイトル：スピネル型鉱物及び関連化合物の高圧相転移
Title : High-pressure phase transitions of spinel-type minerals and related compounds
海洋地殻を構成する中央海嶺玄武岩（MORB）やその上の海洋底堆積物は、スラブの沈み込みに伴ってマントル深部へ運ばれる。下部マントル条件で安定なMORBと堆積物の高圧相の中に、スピネルと同じAB2O4ストイキオメトリーを持つ複数の相が出現する。これらはカルシウムフェライト型相やNAL相と呼ばれており、特徴的な一次元的構造を持っている。最初に、MgAl2O4、MgCr2O4などスピネル型鉱物の高圧相転移に関する実験結果を述べ、これらがスピネル型からカルシウムフェライト型などのポスト・スピネル型相に直接転移するのではなく、中間の圧力下で二相に分解し、新規相が出現することを示す。それに関連して、オフィオライト中の“超高圧クロミタイト”のマントル循環説について議論する。また、MgAl2O4－NaAlSiO4系などの2成分系では、中間組成でカルシウムフェライト型とは異なる六方晶系のNAL相が安定になる。この相は下部マントルにおけるK、Naの重要なホスト相である。これらの2成分系の安定関係に関する最近の実験結果を述べ、さらにキンバライト中のダイヤモンドに包有物として含まれるアルカリ含有鉱物の成因についても議論する。