کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
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4700909 | 1637741 | 2007 | 21 صفحه PDF | دانلود رایگان |
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.
Journal: Chemical Geology - Volume 244, Issues 3–4, 15 October 2007, Pages 584–604