Sulfur speciation in dacitic melts using X-ray absorption near-edge structure spectroscopy of the S K-edge (S-XANES)

Consideration of radiation-induced changes and the implications for sulfur in natural arc systems

verfasst von: Jackie M. Kleinsasser, Brian A. Konecke, Adam C. Simon, Paul Northrup, Antonio Lanzirotti, Matthew Newville, Camelia Borca, Thomas Huthwelker, Francois Holtz
Abstract: The synchrotron technique of micro X-ray absorption near-edge structure spectroscopy at the sulfur K-edge (S-XANES) provides a unique opportunity to measure the proportion of different oxidation states of sulfur (S) in silicate glasses. Although applied extensively in the analysis of basaltic silicate glasses, few S-XANES studies have investigated variations in S oxidation states with f_O2 in felsic silicate glasses. In addition, no study has systematically compared the S-XANES results obtained from the same samples at different photon flux densities to quantify the relationship between exposure time and changes in S speciation in silicate glass, as has been done for Fe and V. This study evaluates observed differences in S speciation measured in experimentally produced H₂O-saturated dacitic glasses over a range of reducing to oxidizing conditions (from log f_O2 = ΔFMQ-0.7 to ΔFMQ+3.3; FMQ is the fayalite-magnetite-quartz mineral redox buffer) and equilibrated at 1000 °C and 300 MPa. S-XANES spectra were collected at three different photon flux densities using three microspectroscopy beamlines. As is observed in S-XANES analyses of basaltic silicate glasses, beam-induced changes to the S⁶⁺/ΣS are observed as a function of photon flux density and beam exposure time. Our results demonstrate that silicate glasses of dacitic composition undergo beam-induced photo-reduction in samples equilibrated at ΔFMQ > +1.75 and photo-oxidation if equilibrated at ΔFMQ < +1. The time required to observe beam-induced changes in the spectra varies as a function of flux density, and our study establishes an upper photon density limit at ~1.0 × 10¹² photons/µm². The S⁶⁺/ΣS calculated from spectra collected below this absorbed photon limit at intermediate flux densities (~1–4 × 10⁹ photons/s per µm²) are affected by beam damage, as no conditions were found to be completely free of beam-induced changes. However, the S⁶⁺/ΣS ratios calculated below the limit at intermediate flux densities are consistent with thermodynamic constraints, demonstrating that S⁶⁺/ΣS ratios calculated from S-XANES spectra can be considered reliable for estimating the oxygen fugacity. Our results carry important implications for the S budget of felsic magmas and dissolution mechanisms in evolved melts. While our results from all three flux densities show the presence of S⁴⁺ dissolved in relatively oxidized (ΔFMQ > +1.75) dacitic glass, even in the spectra exposed to the lowest photon densities, we are unable to rule out the possibility that the S⁴⁺ signal is the result of instantaneous X-ray irradiation induced beam damage using S-XANES alone. When our spectra are compared to S-XANES spectra from basaltic silicate glasses, important differences exist in the solubility of S^2– and S⁶⁺ between dacitic silicate melts, pointing to differences in solubility mechanisms as melt composition changes. This study highlights the need for further investigation into beam damage systematics, presence of S⁴⁺, and the solubility mechanisms of different oxidation states of S as silicate melt composition changes.
Organisationseinheit(en): Institut für Mineralogie
Externe Organisation(en): University of Michigan
LLC
Stony Brook University (SBU)
University of Chicago
Paul Scherrer Institut (PSI)
Typ: Artikel
Journal: American mineralogist
Band: 109
Seiten: 1359-1374
Anzahl der Seiten: 16
ISSN: 0003-004X
Publikationsdatum: 08.2024
Publikationsstatus: Veröffentlicht
Peer-reviewed: Ja
ASJC Scopus Sachgebiete: Geophysik, Geochemie und Petrologie
Elektronische Version(en): https://doi.org/10.2138/am-2022-8833 (Zugang: Geschlossen)

BibTeX

@article{aca2d686c33e4741a767c0916fc5a08b,
title = "Sulfur speciation in dacitic melts using X-ray absorption near-edge structure spectroscopy of the S K-edge (S-XANES): Consideration of radiation-induced changes and the implications for sulfur in natural arc systems",
abstract = "The synchrotron technique of micro X-ray absorption near-edge structure spectroscopy at the sulfur K-edge (S-XANES) provides a unique opportunity to measure the proportion of different oxidation states of sulfur (S) in silicate glasses. Although applied extensively in the analysis of basaltic silicate glasses, few S-XANES studies have investigated variations in S oxidation states with fO2 in felsic silicate glasses. In addition, no study has systematically compared the S-XANES results obtained from the same samples at different photon flux densities to quantify the relationship between exposure time and changes in S speciation in silicate glass, as has been done for Fe and V. This study evaluates observed differences in S speciation measured in experimentally produced H2O-saturated dacitic glasses over a range of reducing to oxidizing conditions (from log fO2 = ΔFMQ-0.7 to ΔFMQ+3.3; FMQ is the fayalite-magnetite-quartz mineral redox buffer) and equilibrated at 1000 °C and 300 MPa. S-XANES spectra were collected at three different photon flux densities using three microspectroscopy beamlines. As is observed in S-XANES analyses of basaltic silicate glasses, beam-induced changes to the S6+/ΣS are observed as a function of photon flux density and beam exposure time. Our results demonstrate that silicate glasses of dacitic composition undergo beam-induced photo-reduction in samples equilibrated at ΔFMQ > +1.75 and photo-oxidation if equilibrated at ΔFMQ < +1. The time required to observe beam-induced changes in the spectra varies as a function of flux density, and our study establishes an upper photon density limit at ~1.0 × 1012 photons/µm2. The S6+/ΣS calculated from spectra collected below this absorbed photon limit at intermediate flux densities (~1–4 × 109 photons/s per µm2) are affected by beam damage, as no conditions were found to be completely free of beam-induced changes. However, the S6+/ΣS ratios calculated below the limit at intermediate flux densities are consistent with thermodynamic constraints, demonstrating that S6+/ΣS ratios calculated from S-XANES spectra can be considered reliable for estimating the oxygen fugacity. Our results carry important implications for the S budget of felsic magmas and dissolution mechanisms in evolved melts. While our results from all three flux densities show the presence of S4+ dissolved in relatively oxidized (ΔFMQ > +1.75) dacitic glass, even in the spectra exposed to the lowest photon densities, we are unable to rule out the possibility that the S4+ signal is the result of instantaneous X-ray irradiation induced beam damage using S-XANES alone. When our spectra are compared to S-XANES spectra from basaltic silicate glasses, important differences exist in the solubility of S2– and S6+ between dacitic silicate melts, pointing to differences in solubility mechanisms as melt composition changes. This study highlights the need for further investigation into beam damage systematics, presence of S4+, and the solubility mechanisms of different oxidation states of S as silicate melt composition changes.",
keywords = "beam damage, oxidation states, S-XANES, sulfate, sulfide, Sulfur",
author = "Kleinsasser, {Jackie M.} and Konecke, {Brian A.} and Simon, {Adam C.} and Paul Northrup and Antonio Lanzirotti and Matthew Newville and Camelia Borca and Thomas Huthwelker and Francois Holtz",
note = "Publisher Copyright: {\textcopyright} 2024 Walter de Gruyter GmbH. All rights reserved.",
year = "2024",
month = aug,
doi = "10.2138/am-2022-8833",
language = "English",
volume = "109",
pages = "1359--1374",
journal = "American mineralogist",
issn = "0003-004X",
publisher = "Walter de Gruyter GmbH",
number = "8",
}