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| Titel |
Complexation of Sr in aqueous fluids equilibrated with silicate melts: effect of melt and fluid composition |
| VerfasserIn |
Manuela Borchert, Max Wilke, Christian Schmidt, Kristina Kvashnina |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038909
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| Zusammenfassung |
| At crustal conditions, the fluid-melt partitioning of Sr is mainly controlled by the salinity of
the fluid and the composition of the melt (Borchert et al., 2010). The data show a sharp
increase in the Sr partition coefficient with the alumina saturation index (ASI) to a maximum
of 0.3 at an ASI of 1.05. Because fluid-melt partitioning of a given element depends on its
complexation in the fluid and its incorporation in the melt, these data imply a change
in the Sr speciation at least one of the two phases. For silicate melts, Kohn et al.
(1990) found only small changes in the first coordination shell of Sr in a suite of
melts with various degrees of polymerization, and argued that incorporation of
Sr in the melt should not play a major role in controlling Sr partitioning. For the
aqueous fluid, Bai and Koster van Groos (1999) and Webster et al. (1989) suggested a
control of the Sr partition coefficient by SrCl2 complexes based on the correlation
between partition coefficient and Cl concentration in the fluid after quenching.
Both hypotheses cannot explain our partitioning data. Thus, new information on Sr
complexation is required. Here, we studied the complexation of Sr in peraluminous or
peralkaline melt dissolved in aqueous fluids in-situ at elevated PT conditions using
hydrothermal diamond-anvil cells (HDAC) and X-ray absorption near edge structure
(XANES) spectroscopy. The starting materials were peraluminous or peralkaline glass
and H2O or a chloridic solution. The glass was doped with high concentrations of
5000 or 10000 ppm Sr. We used bulk compositions with 10 to 15 wt.% glass to
ensure that the melt was completely dissolved in the fluid at high PT conditions.
For qualitative evaluation, we analyzed the starting glasses and various crystalline
compounds and standard solutions. The experiments were performed at beamline ID26
at ESRF (Grenoble, France) using a high resolution emission spectrometer and
Si(311) monochromator crystals for high resolution and Si(111) for low resolution
measurements. An overall resolution of 1.98 eV (Si(311)) and 3 eV (Si(111)) was achieved
using a beamsize of 120x400 μm, Si(777) analyzer crystals and a Rowland circle
diameter of 1 m. Strontium XANES spectra of peraluminous and peralkaline starting
glasses show distinct differences in pre-edge, main edge and position of the first
EXAFS maximum which is directly correlated to the distance of Sr and its nearest
neighbor. Therefore, the spectra indicate an effect of the ASI on the local structure
around Sr. Spectra of standard solution at various PT conditions indicate detectable
temperature-dependent changes in the intensity and peak width of the white line.
The XANES spectra of chloridic solutions with peraluminous or peralkaline melt
dissolved differ significantly from each other. The latter are similar to the one of the
peralkaline starting glass, while this is not the case for solutions with peraluminous
melt. The spectra of water with dissolved peralkaline melt distinctly differ from
those using chloridic fluids. In conclusion, the first spectroscopic evidence on Sr
complexation at elevated PT condition indicates significant speciation changes in both
fluid and melt, and suggest ASI-dependent formation of Sr-Si complexes in the
fluids.
Literature
Bai and Koster van Groos (1999), GCA 63, 1117-1131.
Borchert et al. (2010), GCA 74, 1057-1076.
Kohn et al. (1990), CMP 105, 359-368.
Webster et al. (1989), Econ. Geol. 84, 116-134. |
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