Trace and rare earth element geochemistry of phosphate nodules from the lower Cambrian black shale sequence in the Mufu Mountain of Nanjing, Jiangsu province, China
- 作者：Shao-Yong Jiang ; Hai-Xiang Zhao ; Yong-Quan Chen ; Tao Yang ; Jing-Hong Yang ; Hong-Fei Ling
- 关键词：Rare earth elements ; Redox-sensitive trace elements ; Phosphate nodule ; Black shale ; Lower Cambrian ; South China
- 刊名：Chemical Geology
- 出版年：2007
- 期刊代码：12_00092541
- 类别：ear
- 出版时间：15 October 2007
- 卷：244
- 期：3-4
- 页码：584-604
- 文件大小：3161 K
摘要
Phosphate nodules are abundant in the lowermost section of the Lower Cambrian black shale sequence along a NE-trend belt in the Yangtze Platform, south China. In this study, we examined the Neoproterozoic-Cambrian Mufu Mountain section near Nanjing of Jiangsu province, and conducted a detailed trace and rare earth element (REE) study on the phosphate nodules. We employed a step-leaching acid dissolution protocol of 1 M HAc and 1 M HCl, respectively, to separate carbonate from phosphate mineral contributions in the nodules. Both the HAc-leached and HCl-leached fractions of the phosphate nodules show similar symmetrical variations in trace and REE concentrations and ratios. The total REEs and a number of redox-sensitive trace metals such as V, Cr, Mo, Ni abundances show a systematic increase from the core to rim, whereas the U and Mn abundances decrease from the core to rim in most of the nodules. Similar trends are also observed for the Rb/Sr, U/Th, Co/Ni, and V/Ni ratios, but these trends in the HAc-leached fractions are less pronounced than the HCl-leached fractions. All of the HCl-leached fractions for the nodule cores show seawater-like shale-normalized REE patterns, but the rims of the nodules display slightly MREE-enriched and HREE-depleted patterns. We suggest that the phosphate nodules may have formed in a basinal setting beneath a stratified water column during Early Cambrian, and the compositional variations of REEs and redox-sensitive trace elements from the core to rim may record a changing redox condition and fluid mixing during different stages of nodule growth. The cores of the nodules preserve more pronounced negative Ce anomalies and progressively HREE-enriched patterns, which may retain primary Early Cambrian seawater chemistry, whereas the nodule rims record chemistry consistent with changed redox conditions and/or influences from pore fluids such as generated from degradation of organic matter.