Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

Hong-tao He,Chen Zhu,Siting Zhang,Yun Liu

doi:10.1007/s11631-015-0079-x

Abstract

Abstract Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H4SiO4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments. The results reveal that, in comparison to aqueous H4SiO4, heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of 28Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly 28Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth’s surface systems can be explained. For example, the change of bulk soil δ30Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil δ30Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ30Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadra-coordinated organosilicon complexes and the H4SiO4 solution may also shed light on the Si isotope distributions in the Si-accumulating plants.

Highlights

The silicon element, only second in abundance to oxygen in Earth’s crust, is one of the most dominant constituents in silicates
Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H4SiO4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments
The results reveal that, in comparison to aqueous H4SiO4, heavy Si isotopes will be significantly enriched in secondary silicate minerals

Summary

Introduction

The silicon element, only second in abundance to oxygen in Earth’s crust, is one of the most dominant constituents in silicates. Since Si is a typical non-redox element and often exists in a stable tetrahedron, the variations of its isotopic composition in Earth’s various surface systems are small, no more than 11.8 per mil (Opfergelt and Delmelle 2012). Acta Geochim (2016) 35(1): many of Earth’s surface systems, including soils, rivers, oceans, silicon-accumulating algae and high plants, have all been investigated (Basile-Doelsch et al 2005; Ding et al 2008, 2009; Georg et al 2007, 2009; Hughes et al 2013; Ziegler et al 2005). A concave pattern was identified in the Si-accumulating plants including rice, bamboo, and bananas: the d30Si values decrease from root to stem and increase from stem to shoots (Ding et al 2008, 2009; Georg et al 2009; Ziegler et al 2005)

Results

Discussion

Conclusion