日本地球惑星科学連合2018年大会

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[EE] 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-CG54] ハードロック掘削 オマーンから海洋リソスフェア,島弧形成+

2018年5月21日(月) 15:30 〜 17:00 A09 (東京ベイ幕張ホール)

コンビーナ:高澤 栄一(新潟大学理学部地質科学科)、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)、Peter B Kelemen (共同)、Damon A H Teagle (Ocean & Earth Science, National Oceanography Centre Southampton, University of Southampton, SO14-3ZH, Southampton, UK)、座長:道林 克禎阿部 なつ江(海洋研究開発機構 海洋掘削科学研究開発センター)

16:15 〜 16:30

[SCG54-14] Initial studies on the physical property measurement of listvenite, serpentinite and the metamorphic sole from ICDP Oman Drilling Project Hole BT1B

*岡崎 啓史1畠山 航平2阿部 なつ江3Leong James4道林 克禎6Kelemen Peter5Harris Michelle7Oman Drilling Project Phase 1 Science Party (1.海洋研究開発機構高知コア研究所、2.広島大学理学研究科地球惑星システム学専攻、3.海洋研究開発機構海洋掘削科学研究開発センター、4.アリゾナ州立大学、5.コロンビア大学ラモントドハーティー地球観測所、6.静岡大学地球科学科、7.プリマス大学)

キーワード:オマーンオフィオライト、海洋地殻、蛇紋岩、リスべナイト、含水化反応、炭酸塩化作用

We report results on physical property measurements of core samples from the Oman Drilling Project (OmanDP) Hole BT1B drilled at the Samail Ophiolite, Sultanate of Oman. Cores are mainly composed of listvenite and serpentinite above the basal thrust, and greenstone/greenschist below the basal thrust. The basal thrust of the ophiolite is the trace of a Cretaceous subduction zone, where hundreds of kilometers of oceanic crust and overlying sediments were subducted beneath the ophiolite prior to its emplacement on the Arabian continental margin.

During Leg 2 of the Chikyu-Oman expedition, whole-round X-ray CT image, natural gamma radiation (NGR), magnetic susceptibility (MS), P-wave velocity (Vp) and electrical resistivity, and half-round point magnetic susceptibility and color spectroscopy were measured for all sections of cores on board. P-wave velocity, bulk/grain density and porosity of discrete cube samples and thermal conductivity on pieces from the working halves of the split core sections were also measured.

Listvenite, completely carbonated former peridotite, is the main lithology in Hole BT1B also found in some ophiolite bodies. It is characterized by high thermal conductivity, significantly higher than that of all other lithologies from Hole BT1B, and greater than or equal to that of a typical peridotite. This is due to the high thermal conductivity of the constituent minerals in listvenite (e.g., quartz, magnetite, hematite). There is a systematic gradient in the thermal conductivity and redness of the color spectrum in listvenite at the top of the hole. This could indicate more abundant Fe-oxyhydroxide minerals near the top of the hole, and more abundant carbonate minerals and fuchsite, (Cr-mica formed by the hydrothermal metasomatism involving fluids from subducting metasediments and metabasalts similar to the greenschist/greenstone lithologies in the metamorphic sole\ below the basal thrust. Serpentinite intervals were characterized by lower density, P-wave velocity and resistivity, and higher porosity and MS than those of other lithologies in Hole BT1B. NGR is generally low (<1 counts/s, cps, on average) in listvenites and serpentinites above the basal thrust at 196 m in BT1B, except in a few sections where narrow intervals or veins have significantly higher counts (up to ~8 cps), probably related to concentrations of chromian mica. NGR increases up to 38 cps below the basal thrust, due to the presence of high K in the metasediments and metamorphosed alkali basalts in the metamorphic sole.

Relationships between P-wave velocity and the porosity of BT1B cube samples show two different trends. Listvenite and some greenschist samples have a trend similar to that of gabbroes from Holes GT1A and GT2A, while greenschist and greenstone samples show an inverse relationship similar to the trend in serpentinites from Mid-Atlantic Ridge.