臺灣西南海域永安海脊冷泉址沉積物之反差性黃鐵礦組織

Contrasting Pyrite Textures in Sediments from a Cold-Seep Site at Yung-An Ridge, Offshore Southwestern Taiwan

徐達偉(Ta-Wei Hsu) ； 江威德(Wei-Teh Jiang)

《臺灣鑛業》 67卷2期 (2015/06) Pp. 8-25

摘要

臺灣西南海域永安海脊受到間歇性冷泉活動與強烈甲烷厭氧氧化作用影響，MD052911岩心上段及下段沉積物具有高含量黃鐵礦，局部伴隨碳酸鹽結核粒之形成；電子顯微分析顯示黃鐵礦呈現莓狀體、不規則狀細粒集合體、團帶狀散布之他形至自形晶粒及片狀矽酸鹽礦物解理裂隙內他形晶粒等組織，莓狀體具有高比例之顯著蔓生現象，且鐵膨潤石丸粒中亦形成黃鐵礦，這些多樣性黃鐵礦生長形態可出現於同一標本，且與標本深度無系統性關係，顯示微觀地球化學條件之差異性，符合甲烷緩滲流或擴散之特徵。岩心中段沉積物之黃鐵礦含量甚低，偶見黃鐵礦莓狀體與他形晶粒，缺乏甲烷厭氧氧化作用，與岩心上段及下段沉積物　之黃鐵礦產狀形成強烈對比。

關鍵字

黃鐵礦 ； 顯微組織 ； 甲烷厭氧氧化作用 ； 蔓生 ； 臺灣西南海域

並列摘要

A record of temporal cold-seep activities associated with enhanced anaerobic oxidation of methane in the Yung-An Ridge sediments offshore southwestern Taiwan was marked by the occurrence of high pyrite concentrations and local formation of carbonate nodules in sediments from the upper and lower sections of long piston core MD052911. Electron microscopic analysis of the cored sediments indicate that pyrite has a variety of microtextures including framboidal and irregular aggregates of fine-grained crystals, patches of disseminated anhedral to euhedral grains, and anhedral grains enclosed within microfissures of phyllosilicate cleavage. In addition, high percentages of pyrite framboids showing prominent overgrowth and pyrite neoformation within pellets of ferric smectite occur in these sediments. These diversified pyrite growth textures can occur in a single specimen and have no systematic correlation with burial depth. These imply subtle differences in geochemical environment over short distances, in accordance with a migration mode of methane dominated by slow seepage and/or diffusion. In contrast, the middle-section sediments contain much smaller amounts of pyrite with scanty framboidal aggregates and anhedral grains due to a lack of intense anaerobic oxidation of methane.

並列關鍵字

pyrite ； microtexture ； anaerobic oxidation of methane ； overgrowth ； offshore southwestern Taiwan