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| Titel |
Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity |
| VerfasserIn |
Pedro J. Jugo, Max Wilke, Roman E. Botcharnikov |
| 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 |
250037799
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| Zusammenfassung |
| XANES analyses at the sulfur K-edge were used to determine the oxidation state of S in
natural and synthetic basaltic glasses and to constrain the fO2 conditions for the transition
from sulfide (S2-) to sulfate (S6+) in silicate melts. XANES spectra of basaltic samples from
the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a
dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic
sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV,
similar to that of crystalline sulfides, was present in synthetic glasses quenched from
hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide
species with hydration or presence of minute sulfide inclusions exsolved during
quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo,
Philippines, and absarokitic basalts from the Cascades Range, Oregon, U.S.A., showed a
sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses
and crystalline sulfate-bearing minerals such as haüyne. Basaltic samples from the
Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi
Seamount showed unequivocal evidence of the coexistence of S2- and S6+ species,
emphasizing the relevance of S6+ to these systems. XANES spectra of basaltic
glasses synthesized in internally-heated pressure vessels and equilibrated at fO2
ranging from FMQ-1.7 to FMQ+2.7 showed systematic changes in the features
related to S2- and S6+ with changes in fO2. No significant features related to sulfite
(S4+) species were observed. These results were used to construct a function that
allows estimates of S6+/ΣS from XANES data. Theoretical considerations and
comparison of compiled S6+/ΣS data obtained by SKα shifts estimated with electron
probe microanalysis (EPMA) and S6+/ΣS obtained from XANES spectra show
that data obtained from EPMA measurements underestimate S6+/ΣS in samples
that are sulfate-dominated (most likely because of photo-reduction effects during
analysis) whereas S6+/ΣS data from XANES provide a close match to the expected
theoretical values. The XANES-derived relationship for S6+/ΣS as a function of fO2
indicates that the transition from S2- to S6+ with increasing fO2 occurs over a
narrower interval than what is predicted by the EPMA-derived relationship. The
implications for natural systems is that small variation of fO2 above FMQ+1 will have a
large effect on S behavior in basaltic systems, in particular regarding the amount
of S that can be transported by basaltic melts before sulfide saturation can occur. |
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