Genesis of Mg-phyllosilicate occurrences in the Middle Permian marine successions of South China

Abstract Large-scale sedimentary Mg-phyllosilicate occurrences have rarely been reported in deep marine successions worldwide, with few studies reporting the natural transformation of sepiolite into talc, although published laboratory experiments have proven the existence of this mineral transformation at high temperature. Mg-phyllosilicates occur extensively in the Middle Permian deep marine successions in South China. To investigate the genesis of these extensive occurrences, we conducted integrated studies including field trips, thin section observation, filed emission scanning electron microscope (FE-SEM), vitrinite reflectance (Ro), X-ray diffraction mineralogy (XRD), and trace element and rare earth element (REE) concentration analysis. The XRD patterns suggested that in the west of our study zone, talc was the major species of Mg-phyllosilicates with aliettite and Mg-smectite being minor species, while in the east of our study zone, sepiolite was the major species of Mg-phyllosilicates. Mg-smectite was dominant in the intermediate zone. Ro values serve as a proxy for the maximum paleogeothermal temperature coupled with the differential distribution of these Mg-phyllosilicates. The distribution zones of talc, Mg-smectite and sepiolite correspond to distinct ranges of Ro   1.2%, respectively. This pattern indicates that the maximum paleogeothermal temperature is the controlling factor for natural transformation between Mg-phyllosilicates. Moreover, there are lower transformation temperatures from sepiolite to Mg-smectite (approximately 140 °C) and from Mg-smectite to talc (approximately 155 °C) under natural conditions than revealed in labs. Regarding the cause of original Mg-phyllosilicate deposition, analyses of trace element and rare earth element concentrations reveal that a diffuse Si-rich hydrothermal vent fluid contributed to the elevated Si content in seawater and that the upwelling current extended the Mg-phyllosilicate deposition areas from the deep basin onto the carbonate ramp in the Middle Permian in South China. Hence, favorable seawater conditions for Mg-phyllosilicate deposition occurred in the deep basin and carbonate ramp in the Middle Permian in South China.