Acta Petrologica et Mineralogica

Aquamarine belongs to the beryl group minerals and is produced in granite pegmatites. Its crystallization process can run through different stages of the evolution of pegmatitic magma, and it is also an important host mineral for magma melts and fluid inclusions. Kujierte pegmatite is an LCT type pegmatite that can produce high-quality aquamarine among numerous pegmatite veins in the Koktokay area of Altay, Xinjiang. This article investigates the main, trace elements, and fluid inclusions of aquamarine produced in Kujierte pegmatite. Fluid inclusion petrology shows that the distribution of melt/melt fluid inclusions (MI/M-FI) and fluid inclusions (FI) in aquamarine exhibits a certain pattern: MI/M-FIs are mainly concentrated from the root to the middle of the crystal, while FIs are enriched in the edge region, indicating that the crystallization process of the aquamarine has undergone a relatively complete stage of pegmatite magmatic evolution. The temperature measurement results show that the homoge-nization temperature of M-FIs is >550℃, and the homogenization temperature range of FIs is 220~400℃, with salinity ranging from 0 to 14% and density ranging from 0.6 to 0.9 g/cm². The fluid belongs to H₂O-NaCl-CO₂ system with the medium high temperature and medium low salinity. Based on the orientation of crystal crystallization and the distribution of fluid inclusions, the magma hydrothermal transition stage and post magmatic hydrothermal stage were divided. Electron probe analysis (EPMA) and laser ablation plasma mass spectrometry (LA-ICP-MS) analysis results showed that Fe element dominated the coloration of aquamarine. During the growth process of aquamarine, Fe²⁺ entered the mineral lattice in a channel octahedral substitution mode of Na⁺+Fe²⁺→ Al³⁺, resulting in a light blue coloration. In the early to middle stages of the evolution of pegmatites, the content of alkali metals (Li, Na, K, Cs) and transition metal (Fe, Mg, Mn) elements in aquamarine remained stable, and in the late stage, the content of these metal elements increased sharply. Combined with the long-term compression and subduction of plate in the region, which has led to the overflow of deep material in the earth, and the evolution of pegmatite magma in an unclosed system, it is believed that the fluid enriched with Fe, Mg, and Mn in the deep magma chamber was injected in a pulsating manner in the late stage, resulting in a fluctuating change in the composition of the original hydrothermal fluid.

Kujierte pegmatite in Xinjiang; aquamarine; LA-ICP-MS; fluid inclusion; magma-fluid evolution