Abstract

The iron oxide concretions of Shankargarh (Allahabad), India belongs to Dhandraul Sandstone of Vindhyan Supergroup. Petrography of concretions shows abundant quartz grains embedded within the iron oxide cementation. XRD analysis of the concretion shows diagnostic peaks for quartz, hematite, and goethite. The A–CN–K and A–CNK–FM ternary diagrams drawn for concretion and host rock bulk composition clearly indicate the interaction of concretions rock with iron-bearing diagenetic fluids. A negative Ce anomaly, lower Th/U ratio, and enrichment of redox responsive trace element (e.g., vanadium) indicate concretion formation is redox-controlled. The concretions show Fe enrichment and Si depletion as compared to the host sandstone. The mass balance calculations indicate that the total Fe₂O₃ in the ferruginous sandstone system is 17.63 wt%. The iron mobilization and recycling in the sandstone pore spaces have formed concretions with Fe₂O₃ (25–35% by volume). The sandstone volume required to produce a 6 mm diameter iron oxide concretion is 1807.83 mm³. The Fe laminae and random red colouration patterns in Dhandraul sandstone are consistent with the movement of iron-enriched fluid through pores and spaces. These iron oxide concretions have similarities with the hematite spherules discovered in the Burn Formation, Meridiani Planum, Mars.

Highlights

• Random red colouration pattern, iron laminae, the role of advective and diffusive processes during the formation of the iron oxide concretions present in Dhandraul sandstone are discussed in detail.

• Mineralogical and textural study of the Shankargarh iron oxide concretions using petrography and XRD.

• Fe bearing paleofluid circulation, redox processes, and elemental mobility (enrichment and depletion of elements) are discussed in detail using whole-rock geochemistry of Shankargarh iron oxide concretions and associated sandstone.

• The similarities and difference between Shankargarh iron oxide concretions and Martian hematite spherules.

中文翻译：

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了解存在于Dhandraul（Vindhyan）砂岩中的氧化铁凝结物的成因：对火星赤铁矿小球形成的影响

摘要

印度香卡（阿拉哈巴德）的氧化铁矿属于Vindhyan Supergroup的Dhandraul砂岩。固结岩的岩相学表明，氧化铁胶结层中嵌有大量石英晶粒。XRD的固结物分析显示出石英，赤铁矿和针铁矿的诊断峰。为固结和宿主岩块组成绘制的A–CN–K和A–CNK–FM三元图清楚地表明了固结岩与含铁成岩流体的相互作用。负Ce异常，较低的Th / U比以及氧化还原反应性痕量元素（例如钒）的富集表明固结形成受氧化还原控制。与主体砂岩相比，这些固结物显示出铁富集和硅耗竭。质量平衡计算表明，总Fe ₂ O ₃在铁质砂岩体系中的含量为17.63重量％。砂岩孔隙空间中的铁动员和循环利用已经形成了与Fe ₂ O ₃（占体积的25-35％）的固结体。产生直径为6 mm的氧化铁固结所需的砂岩体积为1807.83 mm ³。Dhandraul砂岩中的铁层和随机红色着色模式与富铁流体在孔隙和空间中的移动一致。这些氧化铁凝结物与火星Meridiani Planum的Burn层中发现的赤铁矿球具有相似之处。

强调

• 详细讨论了红色随机图案，铁片层，在Dhandraul砂岩中存在的氧化铁凝结物形成过程中对流和扩散过程的作用。

• 使用岩石学和XRD对Shankargarh氧化铁凝结物进行矿物学和组织学研究。

• 利用Shankargarh氧化铁凝结物和相关砂岩的全岩石地球化学，详细讨论了含铁古流体循环，氧化还原过程和元素迁移（元素富集和耗尽）的问题。

• Shankargarh氧化铁凝结物与火星赤铁矿球之间的异同。