日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

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-CG49] ハードロック掘削科学~陸上掘削から深海底掘削、そしてオマーン~

2019年5月28日(火) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:道林 克禎(名古屋大学大学院環境学研究科地球環境科学専攻地質・地球生物学講座岩石鉱物学研究室)、高澤 栄一(新潟大学理学部地質科学科)、秋澤 紀克(東京大学 大気海洋研究所 海洋底科学部門)

[SCG49-P05] Olivine fabric and mineral compositions in the Maqsad area of the Samail massif, the southern Oman ophiolite

*高橋 果朋1高澤 栄一2,3道林 克禎4柿畑 優季4仙田 量子5 (1.新潟大学大学院自然科学研究科、2.新潟大学理学部地質科学科、3.海洋研究開発機構海洋掘削科学研究開発センター、4.名古屋大学、5.九州大学)

キーワード:オマーンオフィオライト、結晶方位定向配列、かんらん石、ファブリック、かんらん岩、マントル流

In this study, we conducted structural and chemical analysis of oriented samples of 17 harzburgites and 4 dunites collected from the Maqsad area of Samail massif in the southern Oman ophiolite. We investigated the degree of partial melting and the mantle-melt reaction with relation to mantle diapir structure.

The crystallographic preferred orientation (CPO) of olivine grains were measured in highly polished XZ thin sections using a scanning electron microscope equipped with an electron-back scatter diffraction system at Shizuoka University. After plotting the results in the Vp Flinn plot expanded for olivine fabric types [1,2], the fabric type of peridotites in this study are summarized as follows. The distribution of A type is limited in the southeastern part where the diapir structure has been identified [3], whereas D type is dominant in the peripheral part of the diapir. Minor amounts of AG type and E type are also observed in the peripheral part. From these, the olivine in the southeastern part (diapir area) of this study area is likely to have been deformed under high temperature whereas the peripheral part was deformed at slightly lower temperature. The CPOs of olivine in both areas do not show any influence from the presence of H2O except for only one sample with E type CPO. Moreover, strong alignment of [100] to the vertical orientation was observed in the southeastern part (diapir area) of this study area. Since vertical mantle flow has been estimated in this diapir area, our results are consistent with the previous study [3,4].

The Cr# (=Cr/[Cr+Al] atomic ratio) of spinel in the harzburgites and dunites are as high as 0.55-0.58 in the central part of the southeastern part (diapir area) and along the paleo-ridge axis running in the NW - SE direction [3,4]. On the other hand, the Cr# of spinel in the peripheral part is 0.41-0.55 indicating slightly less depleted than in the central part. In addition, the REE abundance of clinopyroxenes in the central part of the diapir area is lower than in the peripheral part being consistent with the distribution of spinel Cr#.

We consider that the upwelling rate in the southeastern part (diapir area) may have been greater than the peripheral part possibly due to higher temperature in the focused vertical mantle flow. This model is consistent with the distribution of the Cr# of spinel: the degree of melting was greater in the diapir area where Cr# of spinel are higher than the peripheral part. The strongly focused pipe-like model for axis location [3,5] is well fitted by our observations in this study.



[1] Karato et al., 2008, Ann. Rev. Earth Planet. Sci., 36, 59-95. [2] Michibayashi, 2015, J. Geography, 124, 397-409. [3] Jousselin et al., 1998, JGR, 103, 18153-18170. [4] Nicolas et al., 2000, Marine Geophys. Res., 21, 147-179. [5] Nicolas and Boudier, 1995, JGR, 100, 6179-6197.