A refined model for the mechanisms of Precambrian phosphorite formation

IF 4 1区 地球科学 Q1 GEOGRAPHY, PHYSICAL Global and Planetary Change Pub Date : 2024-08-30 DOI:10.1016/j.gloplacha.2024.104562
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Abstract

Despite the economic and scientific importance of Precambrian phosphorites, our understanding of the mechanism leading to their formation remains limited, including for the largest phosphogenic episode in the late Neoproterozoic. To improve our understanding of Precambrian phosphorite formation, we combined sedimentology, petrography, and elemental, and Fe-C isotopic analyses to study the two main phosphorite beds (the lower and upper phosphorite beds) in the Ediacaran Doushantuo Formation, Zhangcunping area, South China. The phosphorites consist mainly of granular textures characterized by densely packed grains, some of which are coated with secondary phosphate growth. However, there are notable differences in the mineralogy, microfossil assemblages, and elemental contents of the two beds. The lower phosphorites have no Ce anomaly, and relatively low Y/Ho ratios and positive δ56Fe values (0.04–0.30 ‰, average of 0.19 ‰). In contrast, the upper phosphorites have negative Ce anomalies and higher Y/Ho ratios and near-zero δ56Fe values (−0.29–0.19 ‰ (average of −0.01 ‰). These observations suggest that the lower phosphorites formed in anoxic-suboxic environments, whereas the upper phosphorites formed in relatively oxygenated environments. The δ13Ccarb values of the phosphorites range from −3.97 ‰ to 1.71 ‰ (average of −1.56 ‰), and are lighter than values in dolostones (−0.52 ‰ to 4.39 ‰, average of 2.02 ‰). This suggesting that formation of the Doushantuo phosphorites was influenced by degradation of organic matter in an ocean with high primary productivity. The lower phosphorites, which were also regulated by Fe redox pumping, have positive δ56Fe values, along with the presence of pyrite framboids and iron oxides, suggesting deposition near the Fe-redox boundary where extensive Fe cycling. The upper phosphorites show positive correlations between Mn and Fe, and Mn/Fe and P2O5, suggesting formation near the Mn boundary with extensive Mn cycling, primarily mediated by Mn redox pumping. Sedimentological observation indicate that primary phosphates were concentrated into granular phosphorites by winnowing processes following primary precipitation. Accordingly, we propose a refined model for Precambrian phosphorite formation in which degradation of organic matter, Fe and Mn pumping, and physical reworking of deposits co-evolve and interact within a dynamic Precambrian redox environment. Our model provides a reasonable explanation for the distribution of global phosphorite deposits throughout geological history.

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前寒武纪磷酸盐岩形成机制的完善模型
尽管前寒武纪磷酸盐岩具有重要的经济和科学价值,但我们对其形成机制的了解仍然有限,包括对新近纪晚期最大的磷酸盐形成过程的了解。为了加深对前寒武纪磷酸盐岩形成机制的认识,我们结合沉积学、岩相学、元素和Fe-C同位素分析,对华南张村坪地区埃迪卡拉斗山坨地层的两个主要磷酸盐岩床(下磷酸盐岩床和上磷酸盐岩床)进行了研究。磷酸盐岩主要由颗粒状纹理组成,其特征是颗粒密集,其中一些颗粒被次生磷酸盐包裹。然而,两个岩床在矿物学、微化石组合和元素含量方面存在明显差异。下部磷酸盐岩没有 Ce 异常,Y/Ho 比值相对较低,δ56Fe 值为正值(0.04-0.30 ‰,平均 0.19 ‰)。相反,上部磷酸盐岩的Ce异常为负值,Y/Ho比值较高,δ56Fe值接近零(-0.29-0.19‰,平均值为-0.01‰)。这些观察结果表明,下部磷酸盐岩形成于缺氧-亚缺氧环境中,而上部磷酸盐岩形成于相对含氧的环境中。磷酸盐岩的δ13Ccarb值在-3.97‰至1.71‰之间(平均值为-1.56‰),比白云岩的值(-0.52‰至4.39‰,平均值为2.02‰)要轻。这表明,豆山沱磷矿的形成是受初级生产力高的海洋中有机质降解的影响。下部磷酸盐岩也受到铁氧化还原泵的调节,其δ56Fe值为正值,同时还存在黄铁矿框架体和铁氧化物,表明沉积在铁氧化还原边界附近,铁循环广泛。上部磷酸盐岩的锰与铁、锰/铁与 P2O5 之间呈正相关,这表明磷酸盐岩是在锰边界附近形成的,主要通过锰氧化还原泵介导了广泛的锰循环。沉积学观察表明,原生磷酸盐在原生沉淀后通过绞碎过程浓缩成颗粒状磷酸盐。因此,我们提出了一个前寒武纪磷酸盐岩形成的完善模型,在这个模型中,有机质降解、铁和锰泵作用以及沉积物的物理再加工在动态的前寒武纪氧化还原环境中共同演化并相互作用。我们的模型为整个地质历史中全球磷酸盐矿床的分布提供了合理的解释。
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来源期刊
Global and Planetary Change
Global and Planetary Change 地学天文-地球科学综合
CiteScore
7.40
自引率
10.30%
发文量
226
审稿时长
63 days
期刊介绍: The objective of the journal Global and Planetary Change is to provide a multi-disciplinary overview of the processes taking place in the Earth System and involved in planetary change over time. The journal focuses on records of the past and current state of the earth system, and future scenarios , and their link to global environmental change. Regional or process-oriented studies are welcome if they discuss global implications. Topics include, but are not limited to, changes in the dynamics and composition of the atmosphere, oceans and cryosphere, as well as climate change, sea level variation, observations/modelling of Earth processes from deep to (near-)surface and their coupling, global ecology, biogeography and the resilience/thresholds in ecosystems. Key criteria for the consideration of manuscripts are (a) the relevance for the global scientific community and/or (b) the wider implications for global scale problems, preferably combined with (c) having a significance beyond a single discipline. A clear focus on key processes associated with planetary scale change is strongly encouraged. Manuscripts can be submitted as either research contributions or as a review article. Every effort should be made towards the presentation of research outcomes in an understandable way for a broad readership.
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