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

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 delta Si-30 values of dissolved silica measured in the spring water and sampling sites along outflowing streams, covering a temperature range between 20 and 100 degrees C, were relatively constant around +0.2 parts per thousand, whereas the delta Si-30 signatures of associated opaline sinters from the streambeds were between -0.1 parts per thousand and -4.0 parts per thousand, becoming progressively more negative in the downstream parts of the aprons. Here, the deposited sinters represent some of the most Si-30 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 delta Si-30 values of the fluids and sinter deposits are higher and lower, respectively. The resulting values for apparent solid-water isotope fractionation (Delta Si-30(solid-water)) decreased with decreasing temperature from ca. -0.7 parts per thousand at similar to 80 degrees C to -3.7 parts per thousand at similar to 20 degrees C, locally down to -4.4 parts per thousand. 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 parts per thousand 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. (C) 2015 Elsevier Ltd. All rights reserved.