日本地球惑星科学連合2016年大会

講演情報

インターナショナルセッション(ポスター発表)

セッション記号 S (固体地球科学) » S-GC 固体地球化学

[S-GC16] Volatile Cycles in the Deep Earth - from Subduction Zones to the Mantle and Core

2016年5月25日(水) 17:15 〜 18:30 ポスター会場 (国際展示場 6ホール)

コンビーナ:*角野 浩史(東京大学大学院総合文化研究科広域科学専攻相関基礎科学系)、羽生 毅(海洋研究開発機構 地球内部物質循環研究分野)、佐野 有司(東京大学大気海洋研究所海洋地球システム研究系)、Jackson Colin(Geophysical Laboratory, Carnegie Institution of Washington)

17:15 〜 18:30

[SGC16-P02] Hydrogen diffusion experiment in apatite: Effect of different water content

*東 佳徳1伊藤 正一1渡邉 賢2坂口 勲2 (1.京都大学大学院理学研究科、2.国立研究開発法人物質・材料研究機構)

キーワード:apatite, hydrogen, diffusion, SIMS

The calcium phosphate apatites, with general formula Ca5(PO4)3(F,Cl,OH), are very common accessory minerals in the terrestrial samples. Since apatite has F, Cl and OH in anion site, there are many studies to discuss about volatile components and water with apatite [1]. Apatite is also major components on extraterrestrial samples, chondrites, achondrites, Moon and Mars [2]. Therefore, apatite is generally recognized as a ubiquitous mineral in our solar system. Recently, there are many studies focusing on the origin and evolution of water in the Earth and solar system based on hydrogen isotopic compositions of apatite (e.g., [3] [4]). However, it is unclear whether the hydrogen isotopic compositions of apatites correspond to the magmatic water in the apatite crystallization or the diffused hydrogen from external water after apatite crystallization by hydrothermal metamorphism.
Hydrogen diffusion in apatite has been recently reported that hydrogen diffusion is caused by hydrogen exchange reaction between the original OH and the diffused hydrogen from water [5]. The H2O concentration in apatite might be possible to control the hydrogen diffusivity. In this study, we performed hydrothermal diffusion experiment with higher water content of Imilchil apatite than that of Durango apatite (e.g.,H2O: ~800ppm [3]) in order to estimate the hydrogen diffusivity of Imilchil apatite.
Apatite crystal from Imilchil, Morocco (H2O: ~10000 ppm) (e.g., [6]) was used for hydrogen diffusion experiment in this study. Specimens were cut along to c–axis and polished with several grades of diamond. Since apatite originally has hydrogen as OH, we chose 2H2O as diffusion source and 2H was used as a tracer of hydrogen diffusivity in apatite [5]. Specimens were annealed under the same conditions of [5]. H–diffusion coefficients in the samples were determined using depth profiles of 2H concentration of the samples obtained by Cameca ims 4f-E7 secondary ion mass spectrometry (SIMS) at LPS Kyoto University.
The depth profiles of 1H and 2H close to the surface region in Imilchil apatite show that the 1H-2H exchange reactions control the diffusion mechanism, which is consistent with that of Durango apatite [5]. H-diffusion coefficients of Imilchil apatite are higher than those of Durango apatite by a factor of about 2. The activation energy of hydrogen diffusion in Imilchil apatite is consistent with that of Durango apatite, which is similar to that of OH-bearing silicate minerals (e.g., [7]). Hydrogen diffusion coefficients are several orders of magnitude faster than that of other elements (e.g., O and Sr, [8]; Mn and U, [9]). As results, this study indicates that the effect of hydrogen diffusion in apatites during hydrothermal geological events should be considered to estimate the origin of magmatic water from hydrogen isotopic compositions of apatite crystals.
References
[1] McCubbin and Jones (2015) Elements, 11, 183-188.
[2] Webster and Piccoli (2015) Elements, 11, 177-182.
[3] Greenwood et al. (2011) Nature Geosci., 4, 79-82.
[4] Usui et al. (2015) Earth Planet. Sci. Lett., 410, 140-151.
[5] Itoh et al. (2015) Goldschmidt abstract, 2015 1394.
[6] Barnes et al. (2013) Chemical Geology, 337-338, 48-55.
[7] Graham (1981) Contrib. Mineral. Petrol., 76, 216-228.
[8] Farver and Giletti (1989) GCA, 53, 1621-1631.
[9] Cherniak (2005) Chemical Geology, 219, 297-308.