Strontium isotope evolution of Middle Permian seawater in the Sichuan Basin, South China: Possible causes and implications

Abstract

Middle Permian 87Sr/86Sr variations are recorded by the marine carbonates in the Qianfo section and the Yundadi3 well in the Sichuan Basin of the Yangtze Platform, China. Petrographic and geochemical examinations of micritic carbonates confirmed the preservation of near-primary 87Sr/86Sr signatures that can be utilized to construct a reliable high-resolution Sr isotope profile of the equatorial eastern Paleo-Tethys for global correlations. The high 87Sr/86Sr ratios associated with a eustatic regression event in the Late Capitanian (late Middle Permian) imply a predominant contribution from terrigenous materials. We observed extremely low 87Sr/86Sr ratios (ca. 0.7068 to 0.7069) in the 40 m-thick Capitanian interval, confirming the significant chemostratigraphic episode called the Capitanian Minimum. Globally low 87Sr/86Sr values in the Middle Permian were probably caused by weak continental weathering rates, suppressed riverine flux into the ocean during Pangean assembly, and increased hydrothermal flux into the ocean associated with the opening of the Neotethys during the Middle Permian. Based on the 87Sr/86Sr values of micritic carbonates from the Qianfo section and the Yundadi3 well, large amounts of nonradiogenic Sr may have been derived from strong submarine hydrothermal activity related to the Emeishan LIP, and areas closer to volcanic activity were more strongly affected. These combined factors were responsible for the observed low 87Sr/86Sr ratios in the Sichuan Basin.