日本地球惑星科学連合2018年大会

講演情報

[EE] 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-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) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 7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)

[SCG54-P02] The record of hydrothermal alteration in the lower oceanic crust sampled by Oman Drilling Project Holes GT1A and GT2A

Michelle Harris1、*Damon A H Teagle2Barbara Zihlmann2Dominik Mock3Tetsu Akitou7Kentaro Kondo4Jeremy Deans5Laura Crispini6Eichii Takazawa7Peter B Kelemen8Jude Coggon2Phase 1 Science Party Oman Drilling Project (1.School of Geography, Earth and Environmental Sciences, Plymouth University, PL4 8AA, Plymouth, UK、2.Ocean & Earth Science, National Oceanography Centre Southampton, University of Southampton, SO14-3ZH, Southampton, UK、3.Leibniz University of Hannover、4.Akita University、5.Southern Mississippi University、6.University of Genoa、7.Niigata University、8.Columbia University)

キーワード:Oman ophiolite, mid-ocean ridges, hydrothermal alteration

Hydrothermal circulation is a fundamental Earth process that is responsible for the cooling of newly formed ocean crust at mid ocean ridges and imparts a chemical signature on both the crust and the oceans. Recent studies highlight the requirement for deep hydrothermal circulation in the lower crust, but the critical samples necessary to resolve the nature of this deep hydrothermal system are poorly sampled in the ocean basins. The Oman Drilling Project successfully cored 3 boreholes into the lower crust of the Semail ophiolite (Holes GT1A layered gabbros, GT2A foliated gabbros and GT3A dike/gabbro transition). These boreholes have exceptionally high recovery (~100%) and have been characterised using both traditional and novel core description methods. These cores provide a timely and important opportunity to quantitatively characterise the hydrothermal system in the lower oceanic crust.
Hydrothermal alteration in Holes GT1A and GT2A is ubiquitous and manifests as secondary minerals replacing primary igneous phases and secondary minerals precipitated in hydrothermal veins and hydrothermal fault zones. Hydrothermal alteration of the host rock was classified as either background, halos, patches or deformation related. Hydrothermal veins were individually logged and cross cutting relationships were recorded to determine the relative timing of hydrothermal alteration. The total alteration intensity in Hole GT1A ranges between 10 -100%, with a mean alteration intensity of 60%, and shows no overall trend downhole. However, there are discrete depth intervals (on the scale of 30 -100 m) where the total alteration intensity increases with depth. Alteration assemblages are dominated by chlorite + albite + amphibole, with variable abundances of epidote, clinozoisite and quartz. Hole GT1A intersected several hydrothermal fault zones, these range from 2-3 cm up to >1m in size and are associated with more complex secondary mineral assemblages. Hydrothermal veins are abundant throughout Hole GT1A, with a mean density of 37 vein/m. Alteration intensity in Hole GT2A ranges between 6-100%, with a mean alteration intensity of 45%, and is highly variable downhole. Alteration halos and patches are slightly more abundant in Hole GT2A than in Hole GT1A. The secondary mineral assemblage is similar to Hole GT1A, but Hole GT2A has higher abundances of epidote, clinozoisite, quartz, laumontite and iron-oxydroxides. Vein density in Hole GT2A is 61 veins/m. In both holes, cross cutting vein relationships indicate a relative timing from earliest to latest of: amphibole; epidote + zoisite + quartz; chlorite + prehnite + quartz, calcite-laumontite-anhydrite; gypsum.