Silicon isotope fractionation during abiotic silica precipitation at low temperatures: Inferences from flow-through experiments

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…However, the isotope composition of the bSiO 2 seemed to remain constant within the reactive surface layer where most of the dissolution and precipitation occurred, which makes isotopic fractionation during dissolution unlikely. From leaching experiments on older Pleistocene and Pliocene sediments, Tatzel et al (2015) report that the formation of authigenic Al-Si phases should be 2‰ lighter than the pore waters they formed from, in agreement with previous experimental results (Delstanche et al, 2009;Geilert et al, 2014;Oelze et al, 2014Oelze et al, , 2015. However, the authors observed an increase of the Si isotope composition of the preserved SiO 2 during phase transformation from opal-A to opal-CT.…”

“…Laboratory experiments confirm that the combined fractionations associated with important processes (Si adsorption or precipitation) are both rate and temperature dependent (Geilert et al, 2014;Oelze et al, 2014Oelze et al, , 2015Roerdink et al, 2015), conclusions that are corroborated by well-designed field experiments . In general, it is clear that there is no such thing as a single fractionation factor for a given process.…”

“…In shelf sediments the rate is thought to be substantially lower at around 56 µmol Si cm −2 yr −1 , which, however, means that approximately 24% of the total dissolving bSiO 2 re-precipitates in the upper few centimeters of the sediment . Several experimental studies have shown that such Si precipitation is associated with strong enrichment of light Si isotopes up to −4.5‰ in the product although they were not focused on sediment-pore water diagenesis (e.g., Geilert et al, 2014;Oelze et al, 2014Oelze et al, , 2015Roerdink et al, 2015).…”

Silicon isotope systematics of acidic weathering of fresh basalts, Kilauea Volcano, Hawai’i

“…Low-temperature processes, such as secondary mineral precipitation, silica adsorption, and plant uptake, are associated with large Si isotopic fractionations (Li et al, 1994;Opfergelt et al, 2006;Milligan et al, 2004;Geilert et al, 2014), while variations in δ 30 Si in igneous rocks are observable but small (Douthitt, 1982;Savage et al, 2010Savage et al, , 2011. Dissolved Si in fresh surface waters has high δ 30 Si values relative to source bedrock or sediment (De la Rocha et al, 2000;Ding et al, 2004;Georg et al, 2007;.…”

“…In systems in which multiple fractionating processes operated simultanously, attributing Si isotopic signatures to a single fractionating mechanism is difficult and frequently requires the application of additional geochemical tracers. Another unresolved issue is that isotopic fractionation factors associated with precipitation of clays and opals (α precip-aqueous ranging from -0.7‰ to -3.5‰ for the 30 Si/ 28 Si ratio, although this range is strongly affected by kinetic processes (Douthitt, 1982;Geilert et al, 2014Geilert et al, , 2015Li et al, 1994;Milligan et al, 2004). Si isotope fractionation factors are rate dependent (Oelze et al, 2014;Geilert et al, 2015).…”