Elemental distribution pattern and forming mechanism of the two types of phosphates in ferromanganese crust in Western Pacific Ocean and their implications

Most of the Pacific ferromanganese crusts have experienced phosphatization before Miocene, presumably by impregnation of phosphorus-rich component into the old crust section during the period of high productivity, that is called the product of phosphatization events. Most of the previous studies on phosphatization were based on point/line analysis of element contents or isotopes, while few studies have been devoted to the growth structure and 2-dimentional element distribution. In this study, a hydrogenetic ferromanganese crust (MDD53) sampled from the western Pacific Ocean was analyzed by electron probe X-ray microanalysis (EPMA) and micro–X-ray fluorescence scanning (μXRF), generating high resolution quantitative data on element concentrations and 2-dimensional element maps, respectively. The results of EPMA and μXRF reveal that the middle to lower part of the crust was phosphatized. The μXRF map shows that there are two types of phosphates. The first type shows sporadic enrichment of Ca and P in the top part of the crust, accompanied by strong post-depositional alterations, such as broken structure, Fe loss, and relatively Mn enrichment. The EPMA data further shows that trace element enrichment of Co and Ni as well as depletion of Pb in the phosphatized area. This observation reflects different affinity of these elements with Fe and Mn oxides, which is consistent with the hypothetic phosphatization mechanism of organic matter degradation at the crust-seawater interface and the resulting alteration of the preformed crust. The second type of phosphate is characterized by an unreported structure of continuous Ca and P enriched laminae located at the bottom of the crust. The lack of post-depositional alteration of the growth structure suggests that this type of phosphate is of syn-depositional or early diagenetic in origin, which is distinctly different from post-diagenetic alteration. The syn-depositional phosphate may indicate an early stage coprecipitation of phosphate and ferromanganese oxide colloids in a relatively shallow water depth, making it a reliable paleoceanographic indicator for high temporal resolution studies of the Late Cretaceous-Early Cenozoic period.