Geological factors responsible for REY-rich mud in the western North Pacific Ocean: Implications from mineralogy and grain size distributions
Geological factors responsible for REY-rich mud in the western North Pacific Ocean: Implications from mineralogy and grain size distributions
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DOI：
10.2343/geochemj.2.0435
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发表时间：
2016-01-01
影响因子：
0.8
通讯作者：
Kato, Yasuhiro
中科院分区：
文献类型：
--
作者：
Ohta, Junichiro;Yasukawa, Kazutaka;Kato, Yasuhiro
在R/V Kairei号巡航KR13-02期间，在西北太平洋南三岛岛以南约250公里处的海底采集了7个活塞芯；在PC04和PC05芯的某些部分，稀土元素和Y(Sigma Rey)的总含量超过4000ppm。显微镜观察显示，高度REY富集层(Sigma Rey&Gt；类似于2,000 ppm)含有大量的磷酸钙和磷灰石。我们对PC04和PC05岩心的块状沉积物和特殊成分(钙、磷和磷灰石)进行了显微观察和粒度分布分析，以阐明REY异常富集的机制。磷酸钙颗粒的形状表明，它们大多是生物成因的。PC04(7.8-8.6Mbsf)和PC05(2.6-3.6Mbsf)富REY层沉积物的粒度分布呈双峰型，峰值在细粒(约4 mm)和粗粒(约40-80 mm)。钙磷矿和磷灰石是这些稀土富集层的粗粒部分的主要成分。Sigma Rey的总体含量主要受生物磷酸钙含量的控制，众所周知，生物磷酸钙是Rey的浓缩物。因此，生物钙磷酸盐的积累增加是导致REY富集化的原因。磷酸钙和磷灰石颗粒基于体积的累积中值直径似乎与磷酸钙和Sigma Rey的含量成正比。磷灰石颗粒大小的增加表明沉积速率较低，这可能使生物钙磷酸盐在没有被低Sigma Rey组分稀释的情况下积累。此外，磷酸钙和磷灰石的颗粒尺寸增大表明，底流的分选使粗粒生物磷酸钙通过去除细粒颗粒(包括低Sigma Rey组分)而集中在沉积物中。解释高REY富集层的总体特征应考虑多种因素。
Seven piston cores were collected from the seafloor similar to 250 km south of Minamitorishima Island in the western North Pacific Ocean during the cruise KR13-02 of R/V Kairei; in some portions of cores PC04 and PC05, the total contents of rare-earth elements and yttrium (Sigma REY) exceeded 4,000 ppm. Microscopic observations showed that the highly REY-enriched layers (Sigma REY > similar to 2,000 ppm) contained significant amounts of calcium phosphate and phillipsite. We conducted microscopic observations and grain size distribution analyses of bulk sediments and distinctive components (calcium phosphate and phillipsite) from cores PC04 and PC05 to elucidate the mechanism of the anomalous REY enrichment. The shapes of the calcium phosphate grains suggest that they were mostly biogenic in origin. The grain size distributions of bulk sediments from the REY-enriched layers of cores PC04 (7.8-8.6 mbsf) and PC05 (2.6-3.6 mbsf) were bimodal, with peaks at fine (similar to 4 mm) and coarse (similar to 40-80 mm) sizes. Calcium phosphate and phillipsite were the major components of the coarse-grained portions of these REY-enriched layers. The bulk Sigma REY content was mainly controlled by the amount of biogenic calcium phosphate, which is well known to concentrate REY. Thus, increased accumulation of biogenic calcium phosphate was responsible for the REY enrichment. The volume-based cumulative median diameters of calcium phosphate and phillipsite grains appeared to be proportional to the contents of both calcium phosphate and Sigma REY. Increased phillipsite grain size suggests a low sedimentation rate, which may have allowed biogenic calcium phosphate to accumulate without dilution by low-Sigma REY components. In addition, increased grain sizes of calcium phosphate and phillipsite suggest that sorting by a bottom current allowed coarse-grained biogenic calcium phosphate to become concentrated in sediments by removing fine-grained particles, including low-Sigma REY components. Multiple factors should be considered to explain the overall features of the highly REY-enriched layers.