Petrology, geochemistry of Mesoproterozoic siderites in the North China Craton and its implications for planetary habitability

Siderite (FeCO₃) is rare in Mesoproterozoic strata, and the reasons for this remain unclear. However, abundant siderites discovered in the Xiamaling Formation of the North China Craton (NCC) provide valuable insights into the iron and carbon cycles, as well as ecosystem evolution in ancient oceans. In this study, we present new petrographic, elemental, zircon U-Pb dating, and C-O and Re-Os isotopic data for the Xiamaling siderites. The results show that siderites are mainly found in Units IV to VI of the Xiamaling Formation, exhibiting stratiform, nodular, and irregular morphologies. The mineral crystals range from euhedral to subhedral ellipsoidal, rhombohedral, and oolitic, or anhedral rosette-like shapes. The siderite aggregates, with irregular structures, are most abundant in Unit V, showing alternating light and dark rims in backscattered electron images. During the deposition of these siderites, submarine volcanism was active in response to the final breakup of the NCC from the Columbia supercontinent. From Units IV to VI of the Xiamaling Formation, the initial ¹⁸⁷Os/¹⁸⁸Os ratios decreased to nearly 0.2, indicating an increase in mantle-derived magmatism, which supplied iron and carbon, contributing up to 30-40%. This period coincides with the disappearance of positive Eu anomalies and a decrease in the influence of hydrothermal fluids. By Unit V, active rifting triggered the release of methane-rich submarine gas hydrates, leading to the alteration of siderites due to dissimilatory iron reduction, resulting in irregular structures. Anaerobic methane oxidation also occurred, causing the siderites to exhibit lower δ¹³C values. The formation of siderite marks the onset of mantle magmatism prior to the peak of the Large Igneous Province at 1.38 Ga, the intensification of chemical weathering, and an increased supply of nutrient elements, which stimulated primary productivity and triggered transient fluctuations in atmospheric oxygen levels.