Experimental constraints on kinetic and equilibrium silicon isotope fractionation during the formation of non-biogenic chert deposits

Abstract

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.