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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Geilert, S., Vroon, P.Z., Keller, N.S., Gudbrandsson, S., Stefánsson, A., van Bergen, M.J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2015
• Subjects: /dk/atira/pure/sustainabledevelopmentgoals/clean_water_and_sanitation; name=SDG 6 - Clean Water and Sanitation; Geysir; Iceland
• Online Access: https://research.vu.nl/en/publications/ac5ecab0-0cd6-41f4-9070-429469aa6f6c; https://doi.org/10.1016/j.gca.2015.05.043; https://hdl.handle.net/1871.1/ac5ecab0-0cd6-41f4-9070-429469aa6f6c

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 δ 30 Si 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 δ 30 Si 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 30 Si 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 δ 30 Si values of the fluids and sinter deposits are higher and lower, respectively.The resulting values for apparent solid-water isotope fractionation (δ 30 Si solid - 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.