Silicon isotope behavior during silica diagenesis recorded by silica sinters in a geothermal system, Xizang, China

Abstract

Silica sinters formed in geothermal systems represent an ideal geochemical archive for studying silicon (Si) isotope (δ³⁰Si) behavior during early diagenesis. Understanding diagenetic processes, including the preservation of Si isotope compositions, is crucial in the matter of paleo-environmental reconstruction. We conducted an investigation in the Dagejia geothermal system located in Xizang, China. This study presents silica diagenetic sequences that span from active geothermal fluids, transitioning from modern and fresh (MF) opal-A to modern and fresh (MF) chalcedony at temperatures ranging from 70 °C to 85 °C and high alkalinity. During diagenesis, the Si isotope values of chalcedony exhibit significant centimeter-scale variations, ranging from −1.89 ‰ to −0.35 ‰. From the top layer MF opal-A to the bottom layer MF chalcedony, the δ³⁰Si values decrease, and the Δ³⁰Si _{chalcedony-opal-A} increases from −1.65 ‰ at 70 °C to −1.05 ‰ at 85 °C.

In this process, the Si concentration of the pore fluid and the Si isotopic composition of the silica sinter were modeled via C.O model and Rayleigh model, respectively. The results show that the Si concentration of pore fluid is controlled by opal-A precursor dissolution and diagenetic depth. These variations in MF chalcedony are caused primarily by kinetic fractionation and are influenced by the temperature and Si concentration of the pore fluid. In this study, we demonstrate that silica diagenetic processes, coupled with isotopic changes in natural siliceous deposits, heavily depend on specific conditions during the formation of silica sinters. Furthermore, they may effectively preserve the initial information present after the formation of silica sinters. These interlinked controls on Si isotopic compositions are crucial for the application of Si isotopes in cherts for paleo-environmental reconstructions.