Silicon isotope fractionation during silica precipitation from aqueous solutions: Experimental and field evidence

Abstract

Climate conditions during Earth’s earliest history were very different from today. Knowledge about this issue is crucial for understanding the development of life on our planet. Billions of years ago, silica-rich rocks formed on the ocean floor, and a small volume is still preserved in surface exposures. They carry traces that provide insight into the properties of ancient seawater. Measuring the proportions of silicon isotopes in these rocks is considered as a promising approach to determine the temperature of seawater that existed long ago. However, application of silicon isotopes is a relatively new research field, and the interpretation of isotope compositions of old as well as recent deposits is hampered by lack of knowledge on the fractionation behavior of the isotopes. The main objective of this research was to expand insights into the fundamental factors that determine the isotope composition of a silica-rich chemical sediment. Through experiments and measurements on deposits from Icelandic hot springs, it has been determined how and to what extent the isotopes fractionate when silica precipitates from a saturated solution. For the first time the influence of temperature on the fractionation behavior has been demonstrated and could be quantitatively constrained. In addition, evidence is provided that properties of the substrate onto which silica is deposited, transformations of the silica minerals after deposition, and provenance of the silica in a seawater basin, are also important factors that influence the ultimate isotope composition of a silica-rich chemical sediment. The results of this research may have significant implications. They enable us to derive more reliable interpretations when using silicon isotopes as tracers of active processes or environmental conditions at the present-day surface, provide a new guideline for the interpretation of isotope data from ancient silica deposits, and question earlier conclusions on the temperature of Precambrian seawater