臺灣西南海域永安海脊甲烷冷泉沉積物中之自生硫複鐵礦

Authigenic Greigite in Methane-Seep Sediments from Yung-An Ridge Offshore Southwestern Taiwan

徐達偉(Ta-Wei Hsu) ； 江威德(Wei-Teh Jiang) ； 黃克峻(Ko-Chun Huang) ； 陳朝煒(Chao-Wei Chen) ； 李柏澍(Po-Shu Lee)

《臺灣鑛業》 67卷1期 (2015/03) Pp. 11-28

摘要

臺灣西南海域永安海脊為甲烷冷泉作用活躍地區，先前研究發現岩心MD052911沉積物在海床以下10~15公尺深度發現有硫化反應前緣之紀錄；硫酸鹽－甲烷過渡帶之強烈甲烷厭氧氧化作用導致孔隙水中硫酸鹽還原，產生還原態硫化物，向深處擴散，與由深處向上擴散之鐵離子反應，沉澱高濃度酸可萃取硫化物。深度1,483~1,487公分沉積物標本來自該岩心酸可萃取硫化物含量最高峰之處，本研究之X光繞射分析顯示其所含黑色硫化物團塊的主要硫化礦物為硫複鐵礦（greigite; Fe_3S_4），另含有少量四方硫鐵礦（mackinawite; ~FeS）。電子顯微分析顯示此硫複鐵礦生成次微米粒徑晶粒，具有短柱狀和長方柱狀兩種形貌，沿一晶軸方向有程度不等之延伸生長，但兩者同樣均呈現立方八面體晶形，聚集成集合體，常見夾於片狀矽酸鹽礦物解理裂隙之間，局部沿〈100〉平行生長，連接成鋸齒狀長鏈集合體，推測此形成於硫化前緣之硫複鐵礦可能是四方硫鐵礦不完全硫化之產物，透過溶解和再沉澱之機制生成。硫複鐵礦是硫化鐵系統非穩定態早成岩作用中之重要假穩定過渡相，但仍甚少文獻得以明確呈現其於沉積物中之形貌與鑑定特徵，本研究增進吾人對海洋沉積物中自生硫複鐵礦之瞭解。

關鍵字

硫複鐵礦 ； 甲烷冷泉 ； 海洋沉積物 ； 硫化前緣 ； 臺灣西南海域

並列摘要

Previous studies indicated the presence of a sulfidization front record in sediments at 10-15 meters below seafloor at the active methane seepage site MD052911 in the Yung-An ridge area off southwestern Taiwan. The sulfidization front is marked by precipitation of high concentrations of acid volatile sulfide (AVS) arising from reactions between downward diffusion of dissolved sulfide produced by sulfate reduction due to intense anaerobic oxidation of methane in the sulfate-methane transition zone and upward diffusive dissolved iron from depth. This study shows X-ray diffraction data indicating that the iron-sulfide mineral constituent of the black AVS at 1,483-1,487 cm below seafloor (peak AVS) is dominated by greigite (Fe_3S_4 thiospinel) with minor mackinawite (tetragonal FeS). Electron microscopic analyses suggest that the greigite forms aggregates of short and elongated prismatic crystals sub-micrometers in size, showing well-developed cubo-octahedral faces. The prismatic elongation is parallel to one of the〈100〉directions. Small aggregates of greigite crystals are often observed to occur within the fissures of phyllosilicate cleavage and locally show parallel growth along〈100〉, forming long crystal chains with a serrate outline. The greigite was formed as a result of incomplete sulfidization of mackinawite in the sulfidization front, probably involving a dissolution and re-precipitation mechanism. Greigite is an important transitional metastable phase in non-steady state early diagenesis of the iron-sulfide system which is presently still not well understood. This study adds to the limited body of direct observation and characterization of authigenic greigite from marine sediments.

並列關鍵字

greigite ； cold-seep ； marine sediment ； sulfidization front ； offshore southwestern Taiwan