Morphological and Sulfur-Isotopic Characteristics of Pyrites in the Deep Sediments from Xisha Trough, South China Sea

Pyrite is one of the common authigenic minerals in marine sediments. Previous studies have shown that the morphological and isotopic characteristics of pyrite are closely related to the geochemical environment where it is formed. To better understand the formation mechanism of authigenic pyrite, we analyzed the isotopic composition, morphology, and distribution of pyrite in the sediment at 500 m below the seafloor from Xisha Trough, South China Sea. Mineral morphologies were observed by scanning electron microscopy and Raman spectrography. X-Ray computed tomography was applied to measure the particle size of pyrite. The size of pyrite crystals in the matrix sediment mainly ranged between 25 and 65 mu m (av. ca. 40 mu m), although crystals were larger (av. ca. 50 mu m) in the veins. The pyrites had a fine-grained truncated octahedral shape with occasionally well-developed growth steps, which implies the low growth rate and weak anaerobic oxidation of methane-sulfate reduction when pyrite was formed. The delta 34S values of pyrites ranged from +20.8 parts per thousand Vienna-defined Canyon Diablo Troilite (V-CDT) to +33.2 parts per thousand V-CDT and from +44.8 parts per thousand V-CDT to +48.9 parts per thousand, which suggest two growth stages. In the first stage, with the continuous low methane flux, the pyrite possibly formed in an environment with good access to seawater. In the second stage, the pyrites mainly developed in sediment fractures and appeared in veins, probably due to the limited availability of sulfate. The less exposure of pyrite to the environment in the second stage was probably caused by sediment accumulation or perturbation. In this study, an episodic pyritization process was identified, and the paleoenvironment was reconstructed for the sediment investigated.