日本地球惑星科学連合2014年大会

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口頭発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG61_2AM2] 岩石・鉱物・資源

2014年5月2日(金) 11:00 〜 12:45 311 (3F)

コンビーナ:*角替 敏昭(筑波大学生命環境系)、藤永 公一郎(東京大学大学院工学系研究科システム創成学専攻)、三宅 亮(京都大学大学院理学研究科地球惑星科学専攻地質学鉱物学教室)、土谷 信高(岩手大学教育学部地学教室)、座長:藤永 公一郎(東京大学大学院工学系研究科システム創成学専攻)、三宅 亮(京都大学大学院理学研究科地球惑星科学専攻地質学鉱物学教室)

12:15 〜 12:30

[SCG61-P12_PG] 単結晶モナザイトの水熱合成及びモナザイトに対する水の溶解度測定

ポスター講演3分口頭発表枠

*阿部 健康1中村 美千彦1 (1.東北大学理学研究科地学専攻)

キーワード:水熱合成, アクセサリー鉱物, モナザイト年代, メタミクト化, NAMs, フーリエ変換型赤外分光装置

Introduction: The U-Th-Pb dating of accessary minerals such as zircon and monazite is widely applied for various types of rocks [1,2,3]. There has been proposed another method to obtain geochronological information from these minerals: quantifying the degree of metamictization (destruction of crystal structure by radioactive components). It is reported for zircon that the water content (up to 10 wt%) is in proportion to the degree of metamictization, thus to the concentration of radioactive nuclei and geological age[4]. Monazite on the other hand usually undergoes much less metamictization than zircon due to the higher bond strength of P and O compared to that of Si and O; this results in the lower water content in the metamictized monazite. Determination of the structural water content in monazite without radioactive damage is thus necessary to constrain the "initial" water content prior to hydration. The water content bears significance also for better understanding the crystal chemistry of monazite. In this study, we synthesized monazite single crystals at hydrothermal condition and determined the content of structural water as a function of pressure.

Experimental method: The hydrothermal synthesis of monazite was conducted at a temperature of 800 degC and pressures of 1.5, 10 and 15 kbar using a cold-seal pressure vessel and a piston cylinder apparatus. The CePO4 reagent was encapsulated with H2O or H2O-NaCl solution and run for ca. 100 hours. The FT-IR analyses of the obtained monazite single crystals were conducted to determine the concentration of structural OH on the basis of Lambert-Beer's Law. The molar absorption coefficient was estimated by linear calibration curve against the OH stretching vibration wavenumber [5].

Results and Discussion:The broad absorption band was observed at 3100-3600 cm-1 in the crystals synthesized in all the experimental conditions. The water content of synthesized monazite was estimated approximately to be 20-70 ppm, showing no large pressure dependence. FT-IR analyses of pleochroic absorption are on-going to determine the OH dipole orientation within the crystal structure.

References: [1]J. M. Langille, M. J. Jessup, J. M. Cottle, G. Lederer, T. Ahmad, Journal of Metamorphic Geology, 30, 769-791 (2012)
[2]E. Janots, A. Berger, E. Gnos, M. Whitehouse, E. Lewin, T. Pettke, Chemical Geology, 326-327, 61-71 (2012)
[3]T. Imayama, K. Suzuki, American Mineralogist, 98, 1393-1406 (2013)
[4]M. Zhang, E. K. H. Salje, R. C. Ewing, Journal of Physics: Condensed Matter, 14, 3333-3352 (2002)
[5]M. S. Paterson, Bulletin de Mineralogie, 105, 20-29 (1982)