Silicon isotope fractionation during silica precipitation from hot-spring waters: Evidence from the Geysir geothermal field, Iceland

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.