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| Titel |
Silicon isotope fractionation during silica precipitation from hot-spring waters |
| VerfasserIn |
Sonja Geilert, Pieter Vroon, Nicole Keller, Snorri Gudbrnadsson, Andri Stefansson, Manfred van Bergen |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093327
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| Publikation (Nr.) |
 EGU/EGU2014-7951.pdf |
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| Zusammenfassung |
| Hot-spring systems in the Geysir geothermal area, Iceland, have been studied to explore
silicon isotope fractionation in a natural setting where sinter deposits are actively formed over
a temperature interval between 20° and 100°C. The SiO2(aq)concentrations in spring
and stream waters range between 290 and 560ppm and stay relatively constant
along downstream trajectories, irrespective of significant cooling gradients. The
waters are predominantly oversaturated in amorphous silica at the temperatures
measured in the field. Correlations between the saturation indices, temperature and
amounts of evaporative water loss suggest that cooling and evaporation are the main
causes of subaqueous silica precipitation. The δ30Si values of dissolved silica in
spring water and outflowing streams average around +1o probably due to the small
quantities of instantaneously precipitating silica relative to the dissolved amount.
Siliceous sinters, in contrast, range between -0.1oto -4.0o consistent with a
preferred incorporation of the light silicon isotope and with values for precipitated
silica becoming more negative with downstream decreasing temperatures. Larger
fractionation magnitudes are inversely correlated with the precipitation rate, which itself is
dependent on temperature, saturation state and the extent of a system. The resulting
magnitudes of solid-fluid isotopic fractionation generally decline from -3.5oat 10°C to
-2.0oat 90°C. These values confirm a similar relationship between fractionation
magnitude and temperature that we found in laboratory-controlled silica-precipitation
experiments. However, a relatively constant offset of ca. -2.9obetween field and
experimental fractionation values indicates that temperature alone cannot be responsible for
the observed shifts. We infer that precipitation kinetics are a prominent control of
silicon isotope fractionation in aqueous environments, whereby the influence of the
extent of the system on the precipitation rate is a central factor. As an important
corollary, the fractionation behavior during precipitation of silica from saturated
solutions in natural environments should be regarded as a system-dependent feature. |
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