Silicon isotope fractionation at low temperatures in the presence of Aluminum: An experimental approach and application to different weathering regimes

by Marcus Oelze

Institution: Freie Universität Berlin
Department: FB Geowissenschaften
Degree: PhD
Year: 2015
Record ID: 1104792
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000098986


During the weathering of minerals and rocks elements are released into the ambient solution. The isotope ratios potentially trace the way Si is released from Si-bearing solids into soil and (diagenetic) interstitial solutions. They also trace how silica is precipitated into secondary solids from these solutions. In particular, the partitioning of Si isotopes in the presence of Al has not been explored in detail under controlled laboratory conditions and the related Si isotope fractionation factors need to be determined. The determination of these fractionation factors is so important as in virtually all Earth surface reactions, Si being released from primary silicates is accompanied by variable amounts of Al. Crucial in the understanding of Si isotope fractionation in the presence of Al are two processes: 1.) Si isotope fractionation during adsorption onto Al precipitates and 2.) Si isotope fractionation during Si precipitation from solutions in the presence of variable Al concentrations. Si precipitation experiments reveal that during cyclic freeze-thaw of dissolved Si-containing solutions, Si is removed from the solution. In the absence of appreciable amounts of Al this removal is not accompanied by a fractionation of Si isotopes. In contrast if Al is present in these solutions at high concentrations (here 1 mmol/l), Si removal is faster and accompanied by strong Si isotope fractionation favoring the light isotopes in the solids. The conducted adsorption experiments presented reveal that adsorption of monomeric silicic acid onto gibbsite is accompanied by a significant kinetic Si isotope fractionation and that light Si isotopes are preferentially adsorbed. The calculated Si isotope fractionation factors are dependent on the initial Si concentration. High initial Si concentrations result in a strong kinetic Si isotope fractionation during adsorption. This initial kinetic signature begins to re-equilibrate only after ca. two months. Having established the principle fractionation factors in these experiments also the Si isotopic composition of natural samples have been explored, to investigate the dependence of Si isotope fractionation related to soil processes under different kinetic weathering regimes. The Si isotope measurements of the amorphous and clay fraction extracted from soils and saprolites reveal that a strong relationship between the Si isotopic composition of these pools and the regolith residence time of the three different weathering regimes exists. An increase in regolith residence time leads to lower 30Si/28Si ratios for secondary silicates formed in different weathering regimes. An isotope mass balance model reveal that the proportion of particulate export flux increases over the dissolved import Si flux according to the decrease in regolith residence time. This change is mirrored in the 30Si/28Si ratios of secondary precipitates. Während der chemischen Verwitterung von gesteinsbildenden Mineralen und anstehendem Festgestein werden Elemente in…