Characteristics and Genetic Model of Europium (Eu) Negative Anomalies in Rare Earth Elements from the Sepiolite-containing Strata of the Mao-1 Member, Sichuan Basin

Key words: 
• δEu negative anomaly /
• Genetic model /
• Sepiolite-containing successions /
• The Member Mao-1 of Maokou Formation /
• Sichuan basin

Abstract: [Objective] Europium (Eu), a redox-sensitive rare earth element (REE), serves as a proxy for paleo-ocean reconstruction. The Middle Permian Maokou Formation, Member 1 (hereafter Mao-1 Member) in the Sichuan Basin exhibits a characteristic Eu negative anomaly, yet its genesis remains unclear. [Methods] Utilizing integrated approaches including field section measurement, core observation, thin-section petrography, major element, trace element, in-situ micro-scale carbon-oxygen isotope, and rare earth element analyses, the rock types within the Mao-1 Member were classified. The characteristics of the Eu negative anomaly across different lithologies were summarized. Combined with the mechanisms governing REE occurrence and fractionation, a genetic model for the Eu negative anomaly in the sepiolite-containing successions of the Mao-1 Member was established. [Results] The Mao-1 Member primarily consists of rhythmic alternations of mudstone, argillaceous limestone, and limestone. Laminated and lenticular sepiolite predominantly occurs in mudstones and argillaceous limestones, while sporadic sepiolite is common in limestones. All three rock types exhibit REE distribution patterns with Eu negative anomalies, with the anomaly intensity decreasing in the order: mudstone > argillaceous limestone > limestone. Within the cool-water realm, the Eu negative anomaly intensifies with increasing temperature. Vertically, trends in paleoenvironmental proxies (Sr/Cu, ??Sr/??Sr, δ13C, δ1?O) show good synchronicity with δEu. Based on the modes of occurrence and fractionation characteristics, a genetic model for the Eu negative anomaly is proposed. The sepiolite-containing successions of the Mao-1 Member were deposited in a cool-water environment, with paleo-seawater temperatures ranging from 5 to 15°C. In seawater, Eu primarily existed as the carbonate complex [Eu(CO?)]- 2 releasing free Eu3? ions capable of substituting for Ca2? and being adsorbed by both sepiolite and organic matter. [Conclusions] During the limestone depositional stage, with paleo-seawater temperatures ranging from 7.47 to 11.08°C, the low abundance of Eu3? and its low partition coefficient hindered its incorporation into the aragonite lattice, resulting in a Eu negative anomaly. During the argillaceous limestone depositional stage, with paleo-seawater temperatures ranging from 8.37 to 12.62°C, sepiolite precipitated extensively, selectively adsorbing Sm3?/Eu3? to form unstable outer-sphere complexes, while Tb3? was adsorbed to form stable inner-sphere complexes. Furthermore, the quantity of REEs adsorbed by sepiolite far exceeded that incorporated into the carbonate lattice, significantly exacerbating the relative depletion of Eu. During the mudstone depositional stage, with paleo-seawater temperatures ranging from 8.15 to 14.25°C, sepiolite synergistically adsorbed Tb3? with organic matter, further intensifying the Eu negative anomaly. During the diagenetic stage, the stability of the complexes increased, preserving the Eu negative anomaly. Therefore, the Eu negative anomaly in the Mao-1 Member was formed by the differential complexation of REEs by sepiolite within a cool-water environment and was preserved from the depositional stage through the diagenetic stage.