 |
| Titel |
High-field strength elements complexing in aqueous fluids at subduction zone P-T conditions |
| VerfasserIn |
Julien Dubrail, Max Wilke, Christian Schmidt, Karen Appel, Sakura Pascarelli, Kristina Kvashnina, Craig Manning |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048597
|
|
|
|
|
|
| Zusammenfassung |
| One of the characteristic geochemical features of subduction zones is the depletion of
high-field-strength elements (HFSE, i.e., Ti, Zr, Hf, Nb and Ta) relative to large ion
lithophile elements (LILE, i.e., Rb, Sr, Cs, Ba, Pb, U and Th) in primitive arc magmas
compared to the HFSE and LILE concentrations in mid-ocean ridge basalts. The
mechanism that causes the HFSE depletion is still not completely understood. One of
the reasons is that the knowledge on the HFSE speciation in fluids at subduction
zone conditions is only fragmentary. Complexing of HFSE with alkalies and silica
dissolved in aqueous fluids has been suggested as an efficient mechanism to promote
HFSE mobility during fluid–rock interaction in the lower crust and upper mantle
[1-2].
Here, we obtained data on the solubility of Zr and Hf in aqueous fluids containing
Na2Si3O7(NS3), Na2Si3O7 + 1 wt% Al2O3, Na2Si3O7 + 5 wt% Al2O3, NaOH, or HCl. For
this, these fluids were equilibrated with zircon or hafnon in hydrothermal diamond-anvil cells
at temperatures up to 750 Ë C and pressures up to ~1 GPa and analysed in-situ using
synchrotron radiation micro-XRF and XAFS at beamline L HASYLAB, DESY and at the
ESRF beamlines ID 24 and ID26.
The Zr and Hf concentrations in the fluids are highest in aqueous NS3 solutions and
decrease with addition of alumina. Determined Zr and Hf concentrations in NS3 + 5
wt% Al2O3 fluids were about the same or somewhat higher than those in NaOH
and HCl solutions. The enhanced Zr and Hf solubilities in NS3 fluids point to the
formation of (Zr,Hf)-O complexes involving Na and/or Si, i.e., they signify the
importance of dissolved alkalies and silica for the transport of HFSE in subduction-zone
fluids.
Moreover, we performed XANES measurements at the Zr-K and Hf-L3 edges to obtain
more detailed information on the HFSE complexes in aqueous fluids at high pressures and
temperatures. Theoretical ab-initio XANES spectra were calculated using the FEFF9 code [3]
to extract structural data from the experimental spectra. The simulated XANES spectra of
clusters based on the structure of vlasovite (Na2ZrSi4O11) are in good qualitative agreement
with the Zr-K and Hf-L3 XANES spectra in NS3 solutions. This indicates Zr and Hf
complexes with 6 oxygens in the first coordination shell with additional Si and
Na neighbours in further shells. Experimental XANES spectra from Zr and Hf in
HCl and NaOH solutions differ significantly from those of fluids with dissolved
silicate.
In the case of aqueous HCl solutions, a (Zr,Hf)O4Cl3 cluster with a 7-fold first
coordination shell reproduced qualitatively the features of the experimental spectra.
Comparison of the experimental spectra on Zr in NaOH solutions showed good
agreement with experimental and calculated spectra of baddeleyite (ZrO2) model
compound, which points to complexes with 7 oxygens in the first coordination
shell.
References:
[1] Manning, C. E. (2004), Earth Planet. Sci. Lett. 223, 1-16
[2] Manning, C.E. et al. (2008), Earth Planet. Sci. Lett. 272, 730-737
[3] Rehr, J.J., Albers, R.C. (2000), Reviews of Modern Physics. 72, 621-654 |
|
|
|
|
|
|