中印度洋海盆表层沉积物稀土元素分布特征及富集规律

Distribution and enrichment principles of rare earth elements in surface sediments from the Central Indian Ocean Basin

对中印度洋海盆14个站位的表层沉积物进行了稀土元素(REE+Y,简称REY)分布特征和富集规律研究。结果表明,样品中REY主要富集于沸石黏土和远洋黏土中(稀土元素总量最高为1239×10^(−6)),且明显富集钇(Y)等重稀土元素(Y富集系数高达14.1,重稀土元素和Y富集系数最高为11.6);富稀土沉积物呈明显Ce亏损,发育在受南极底层流影响的氧化环境中;鱼牙骨等生物磷灰石是深海稀土富集的重要富集矿物或宿主矿物。样品地球化学特征表明,深海富稀土沉积是有别于已知陆地稀土矿床的一种新类型。研究区沉积物中REY的富集与构造位置(距洋中脊距离)、氧化还原条件、发育水深和沉积物类型等密切相关。初步推测距离东南印度洋中脊450~1200 km范围内、水深超过碳酸盐补偿深度(CCD)的沸石黏土发育区是最具资源潜力的区域;推测在中印度洋海盆北部可能埋藏有一个向北部延伸的富稀土沉积层,其埋藏深度随着远离洋中脊而逐渐加大,富集层厚度可能与暴露在初始富集区域的时间密切相关。

The distribution and enrichment mechanisms of rare earth elements and Yttrium(REY)are investigated based on fourteen surface sediment samples collected from the Central Indian Ocean Basin.The results show that the REY-rich sediments mainly consist of phillipsite-bearing pelagic clay and red pelagic clay,and are characterized by enrichment in heavy rare earth elements(HREY),such as Yttrium.These REY-rich deep-sea sediments developed in oxidizing settings influenced by Antarctic bottom water(AABW)and are characterized by remarkably negative Ce anomalies.Bio-apatite is the main REY enriched mineral or the host mineral in these deep-sea sediments.The results of geochemical analysis indicate that the REY-rich sediment is a new type of deposit,which is different from terrestrial rare earth element deposits.The formation of REY-rich sediment is clearly related to the redox conditions,tectonic location(distance to the mid-ridge),and water depth.We speculate that the potential area of REY-rich sediment is 450 to 1200 km from the Southeast Indian ridge,with a water depth greater than the carbonate compensation depth(CCD).It is speculated that there is a northward-extending REY-rich layer in the northern part of the Central Indian Ocean Basin,with a gradually increasing burial depth to the north from the Southeast Indian mid-ridge.Additionally,the thickness of the enriched layer may be closely related to the residence time in the initial enrichment area.