ALBERT

Description: Silicon isotopic compositions (δ30Si) of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments, but interpretations are hampered by the lack of experimentally validated fractionation factors. Here, we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch-reactors at low temperature (10-35°C) and near-neutral pH (7.5-8.5), as analogue for non-biogenic chert formation. Instantaneous fractionation factors, derived from δ30Si-values of the total dissolved (SiTD) silica and mass balance computations with αinst=(δ30Sippt+1000)/(δ30SiTD+1000), decrease with progressive precipitation and reduced reaction rates. This suggests that silica deposition in the batch-reactors is kinetically-dominated at the start of the experiments but approaches a metastable equilibrium after ca. 400hours. Modelled kinetic fractionation factors range from 0.9965 at 10°C, to 0.9976 at 20°C and 0.9993 at 35°C and pH8.5, whereas equilibrium isotope effects are smaller and range from 0.9995 at 10°C, to 1.000 at 20°C and 1.0005 at 35°C. Our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated, implying that the kinetic conditions of non-biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks, with particular relevance for those preserved in the Archean chert record

Description: This study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the Geysir geothermal area (Iceland), a setting where sinter deposits are actively formed. The δ30Si values of dissolved silica measured in the spring water and sampling sites along outflowing streams, covering a temperature range between 20 and 100 °C, were relatively constant around +0.2‰, whereas the δ30Si signatures of associated opaline sinters from the streambeds were between −0.1‰ and −4.0‰, becoming progressively more negative in the downstream parts of the aprons. Here, the deposited sinters represent some of the most 30Si depleted abiotically produced terrestrial materials documented to date. Compared to the data reported for Icelandic basalts, considered to be the source of the silicon, the δ30Si values of the fluids and sinter deposits are higher and lower, respectively. The resulting values for apparent solid–water isotope fractionation (Δ30Sisolid–water) decreased with decreasing temperature from ca. −0.7‰ at ∼80 °C to −3.7‰ at ∼20 °C, locally down to −4.4‰. This temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid. However, the apparent fractionation magnitudes observed in the field are ca. −2‰ more negative and thus significantly larger. We infer that solid–water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature, but is essentially a function of the precipitation rate, such that the fractionation factor decreases with increasing rate. As an important corollary, the effective fractionation behavior during precipitation of silica from saturated solutions is a system-dependent feature, which should be taken into account when using silicon isotopes for paleo-environmental reconstructions.

Description: Detectible δ30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ30Si signatures. Here, we report δ30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ30Si of − 0.7 ± 0.2‰, while opal-CT samples are isotopically lighter with an average δ30Si of − 2.0 ± 0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.