Yankuang ceshi (Mar 2017)
Study on the Formation Mechanism of Phosphate Nodules in the Early Cambrian Period Using LA-ICP-MS Multi-element Imaging Technology
Abstract
The phosphorus-rich sediments, including phosphate nodules and phosphate rock, at the bottom of the Lower Cambrian, are widespread in China. These sediments are also considered as one of the most important chemical symbol layers in early Cambrian and also indicate the great change of the Earth's surface system in the transitional period of Cryptozoic to Phanerozoic. Phosphorus sources and the enrichment mechanism remains a matter of debate. In order to better understand the depositional mechanism of these phosphorus-rich sediments, Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) was used to perform in situ multi-element imaging in the micro areas of phosphate nodules from the black shale of the Niutitang Formation in the Jinsha area of Guizhou Province. The results show that the enrichment of each element in phosphorus tuberculosis clearly records the trend of the microenvironment during the formation of phosphorus tuberculosis. Ca and P are enriched and Si is depleted in the inner layer of the phosphate nodule, which means that the enrichment of P was due to the formation of authigenic apatite. P is derived from organic matter containing oxygen or anaerobic degradation release. The co-enrichment of Mn, Zn with P in the interior indicates an oxic bottom water layer during the formation of the phosphate nodule. The enrichment of As, Mo, V in the surrounding rocks or black shale implies that the reduced bottom water ended the growth of the phosphate nodule. This study shows that LA-ICP-MS in situ multi-element imaging technology can reveal rich geochemical information in the micro-area, which can provide high-precision visualization evidence, and will be widely used in geological exploration and paleo-environmental research.
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