南沖繩海槽表層沈積物中黏土礦物之研究

Abstract

黏土礦物在海洋研究中普遍被用來當作沈積物的來源與傳輸的指標，沈積物中黏土礦物的種類及分布情形與鄰近區域岩石源區的性質和氣候有關。本研究依照美國地質調查所（USGS）所公布之黏土礦物分析方法流程，使用X光繞射法（X-Ray Diffraction; XRD）鑑定黏土礦物種類並進行半定量分析工作。 研究結果顯示，南沖繩海槽表層沈積物內黏土礦物中伊利石含量最高，其次為綠泥石，高嶺石再次之，膨潤石含量最低。 在含量變化上，伊利石在南沖繩海槽由西向東遞減，結晶度表示其可能為近變質帶岩石所風化而來，主要來源為蘭陽地區所輸入之沈積物。綠泥石與高嶺石於棉花峽谷下緣及海槽中心有相對高值，且與濁流沈積有良好之正相關，顯示其為自陸棚或陸坡所傳輸而來。膨潤石的變化明顯看出與海槽中海底火山的位置有關。Esquevin-Index分析結果顯示海槽內沈積物中富鐵鎂伊利石與富鋁伊利石含量差異不大，幾處較低值可能與其源岩較多黑雲母類（Biotitic）岩石有關。 在南沖繩海槽沈積物樣本及東部各河川樣本中黏土礦物組成上，顯示海槽沈積物來源與陸上蘭陽地區輸出相關性較高，而花東地區河川沈積物供應量的比重可能較低。 本研究認為南沖繩海槽沈積物內綠泥石可能為富含鐵的鮞綠泥石（Chamosite），經由加熱處理方法時並無出現明顯綠泥石繞射峰值。

Clay minerals are one of the most useful indicators marine geology to decipher the source and transport of sediments in oceanography. The composition and distribution of clay minerals in sediments are related to the climate and the nature of their parent rocks. In this study, the analysis result of the clay minerals illite is the most abundant clay mineral in the southern Okinawa Trough (SOT), the chlorite and kaolinite are more in the study area. The amount of smectite is the lowest in the sediment. The amount of illite increases from west to east SOT and the crystallinity of illite shows it might come from the weathering of rocks in the anchizone in the watershed of Lan-Yang River. The amount of chlorite and kaolinite is higher at the Mien-Hwa Canyon and the deepest part of SOT. The good correlation between chlorite, kaolinite and the turbidte distribution suggests that they were transported from the shelf or slope. We also observed that the amounts of smectite in the sediments are related to submarine volcanoes in the adjacent area. According to the Esquevin-Index, there is no difference between the amount of Fe,Mg-rich illite and Al-rich illite in the sediments of SOT. In some areas, the lower Esquevin-Index values indicate that the parent rocks might be more biotitic. Comparing the composition of clay minerals in the SOT and the riverine sediments of the Eastern Taiwan, we found that the Lan-Yang River is the most important source to the sediments in the SOT.