Amagmatic-derived aegirine and its potential sedimentary origin: Evidence from alkaline sediments of Permian formations, Junggar Basin, China

Aegirine, with its potential sedimentary origin, may provide significant information on the diagenetic condition in alkaline deposits. Previous studies have predominantly focused on its igneous petrogenesis, whereas evidence for its low-temperature authigenic origin remains sparse and poorly constrained. Here, we report newly identified aegirine from the Permian alkaline lacustrine strata in the Junggar Basin, northwest China, and present direct fluid-inclusion and multi-isotopic constraints on its origin. Petrography and fluid-inclusion homogenization temperatures (100−130 °C) confirm a low-temperature authigenic origin. This temperature range, approaching the experimental synthesis threshold (∼130 °C), provides the first direct temperature evidence for natural aegirine formation in sedimentary environments. Distinctive Sr-Nd isotopic signatures (εNd = −3.5 to −2) together with coupled δ18O (13.85‰ to 15.20‰) and δ30Si (−1.88‰ to −1.74‰) values rule out a magmatic source, instead supporting its formation via low-temperature sedimentary-hydrothermal processes. δ7Li (6.9‰ to 11‰) and δ56Fe (0.44‰ to 0.56‰) values are consistent with mixing between a late-stage alkaline hydrothermal solution and external fluid at low temperature. Occurrence of aegirine in alkaline lacustrine sequences and its paragenetic relationship with other alkaline minerals indicate that alkaline chemistry enables aegirine crystallization at a relatively low temperature, and reveals a diagenetic sequence from Na-carbonate to Na-silicate phases in an alkaline diagenetic environment.