Hydrothermal activity during Ediacaran–Cambrian transition: Silicon isotopic evidence

Abstract

Abundant bedded chert deposits occurred at the Ediacaran–Cambrian transition interval at the Yangtze Platform, South China. However, there is not a non-controversial and integrated model for the origin of these chert deposits from trace element patterns and oxygen isotopic evidence. To understand the origin and the oceanic environment of these chert deposits, we analyzed the Si isotopic composition, major and trace elements of chert samples from two stratigraphically correlated sections with a depositional age around 542Ma. The relationship between Al2O3 content and Si isotopic values indicated three end-member sources of Si derivation. The most negative δ30Si values (−0.3‰ to −0.5‰) reflect the silicon derived from hydrothermal fluids due to intense tectonic activity. The most positive δ30Si values (up to +1.2‰) could reflect the increase of 30Si in the hydrothermal fluid by precipitation, or the signature of seawater. The medium δ30Si value (+0.2‰ to +0.7‰) could be interpreted as the mixing of the hydrothermal fluid and volcanic materials with negative δ30Si and ambient seawater with positive δ30Si. It is obvious that the injection of Si-rich hydrothermal fluid would affect the δ30Si and trace element distribution of seawater. In addition, we estimated that the δ30Si values of Ediacaran–Cambrian transitional seawater range from +2.2‰ to +3.5‰ based on assumed fractionation factors −2.3‰<ɛ<−1.0‰ between precipitated Si in chert deposits and initial dissolved Si in seawater. According to Si isotopic composition and other geochemical evidence, we maintain that hydrothermal activity played an important role not only in the formation of bedded chert and polymetallic ore deposits, but also in the oceanic environment and concurrent evolution of life during Ediacaran–Cambrian transition interval.