Abstract
Iron formations (IFs) are traditionally considered to be limited during 1.8−0.8 Ga. However, there are recent reports of siderite-dominated IFs within this time interval, such as the 1.40 Ga Xiamaling IF in North China and the 1.33 Ga Jingtieshan IF in Qilian. To further explore the crystallization and formation mechanisms of siderite, an authigenic siderite crystal from the Xiamaling IF was fully scanned using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Multi-element imaging with a spatial resolution of 5 μm revealed an obvious rim structure of the siderite crystal, which might record the crystallization and growth processes. The Al- and Fe-enriched zone in the core of siderite crystal might be an iron-bearing nucleus, and the formation of rim structure was related to the transition from a closed crystallization environment to a semi-closed growth environment. These results, combined with carbon isotope evidence from the siderites and surrounding shales, suggest that vigorous dissimilatory iron reduction that can provide Fe2+ and HCO3− to the pore water is a key factor to form the siderite-dominated Xiamaling IF.
Highlights
Most marine massive iron formations (IFs) were deposited during 3.0–1.8 Ga, with a brief return around 0.8 Ga [1]
Petrographic analysis reveals that most siderite crystals are subhedral to euhedral with a generally brown core and bright rims (Figure 3a,b)
The formation of 12 Cdepleted siderite was attributed to the contribution of organic-sourced HCO3 − through dissimilatory iron reduction (DIR) [6,7]
Summary
Most marine massive iron formations (IFs) were deposited during 3.0–1.8 Ga, with a brief return around 0.8 Ga [1]. During the one billion-year gap between 1.8 and 0.8 Ga, the IFs shortage is thought to be a result of significantly decreased Fe2+ concentration in the ocean, which was either substantially oxidized [2] or sulfurized [3]. Recent studies provide increasing evidence that ferruginous water was widespread in the deep ocean until. Different to the hematite- and magnetite-dominated 3.0−1.8 Ga IFs [9], the Xiamaling and Jingtieshan IFs are dominated by siderite [6,7,8]. In Australia, massive siderite deposits have been found in the ~1.45 Ga Sherwin ironstone [10] and 2.5 Ga Hamersley IF [11]
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