Understanding genesis of iron oxide concretions present in Dhandraul (Vindhyan) Sandstone: Implications in formation of Martian hematite spherules

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.