Multi-Element Imaging of a 1.4 Ga Authigenic Siderite Crystal

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.

Early Diagenesis in the Lacustrine Ostracods from the Songliao Basin 91.35 Million Years Ago and Its Geological Implications

“…However, during the burial and early diagenetic stage, a series of heterotrophic microbial processes occurs to degrade organic matters and generate secondary carbonate minerals, such as dolomite, calcite, ankerite, and siderite [18][19][20][21]. As the bicarbonate (HCO3 − ) forming these secondary minerals is mainly from pore water rather than water column [19,22], the minerals are encased in ostracod shells and may be difficult to effectively remove, thereby causing geochemical contamination. Therefore, it is necessary to identify and evaluate the early diagenesis in the ostracods.…”