Abstract Spherical carbonate concretions are present in sedimentary strata of varying geological ages worldwide. Recent studies reveal that calcium carbonate concretions form very rapidly around dead organisms after burial in the seabed. However, the formation mechanism of spherical dolomite concretions in marine sediments, particularly the carbon source and the reason for their spherical shape, are still moderately known. This study aims to elucidate their formation process and diagenetic evolution through the characterization of the structure, mineralogical composition, and geochemistry in and around a concretion. Here, detailed studies were conducted on a gigantic dolomite concretion approximately 170 cm in diameter, which formed in tuffaceous fine sandstone of the Morozaki Group in Chita Peninsula, Japan. The Ca and Mg distributions in and around the concretion show that it rapidly formed by outward diffusion of bicarbonate from the carbon source in its center. The δ13C values ranging from +4.4‰ to +7.5‰ and large volume of dolomite cement indicate that the dolomite concretion formed at shallow depth from methanogenic organic matter decomposition during rapid sedimentation. Heulandite occurred only in the surrounding rock matrix comprising altered volcanic glasses because of high temperature during deep burial up to 2–4 km depth. This gigantic dolomite concretion properly preserves its evolution and changes in the superimposed post-depositional environment. This study shows that gaining a better understanding of spherical dolomite concretions can potentially help reveal the burial process of sediments during early diagenesis.

中文翻译：

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海洋沉积物中巨大球形白云石结核的快速形成

摘要 球形碳酸盐结核存在于全球不同地质时代的沉积地层中。最近的研究表明，在海底埋葬死生物后，碳酸钙结石会在死生物周围迅速形成。然而，对于海洋沉积物中球形白云石结核的形成机制，特别是碳源及其球形原因，目前尚不清楚。本研究旨在通过对结核内部和周围的结构、矿物组成和地球化学的表征来阐明它们的形成过程和成岩演化。在这里，对日本知多半岛 Morozaki Group 凝灰质细砂岩中直径约 170 cm 的巨大白云石结核进行了详细研究。结核内部和周围的 Ca 和 Mg 分布表明，它是由碳酸氢盐从其中心的碳源向外扩散而迅速形成的。δ13​​C 值在+4.4‰～+7.5‰ 范围内，白云石胶结物体积较大，表明白云石结核是在快速沉积过程中产甲烷有机质分解形成的浅层白云石结核。由于深埋达 2-4 公里深度时的高温，辉沸石仅出现在包含蚀变火山玻璃的围岩基质中。这个巨大的白云岩结核在叠加的沉积后环境中适当地保存了它的演化和变化。这项研究表明，更好地了解球形白云石结核可能有助于揭示早期成岩作用中沉积物的埋藏过程。