Electronic environments of ferrous iron in rhyolitic and basaltic glasses at high pressure. Issue 8 (31st August 2017)

• Record Type: Journal Article
• Authors: Solomatova, Natalia V.; Jackson, Jennifer M.; Sturhahn, Wolfgang; Rossman, George R.; Roskosz, Mathieu
• Is Part Of: Journal of geophysical research. Volume 122:Issue 8(2017)
• Page Start: 6306
• Page End: 6322
• Publication Date: 2017-08-31
• Subjects: silicate glass -- silicate melt -- Mossbauer spectroscopy -- high pressure -- coordination environment -- ferrous iron; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2017JB014363
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The physical properties of silicate melts within Earth's mantle affect the chemical and thermal evolution of its interior. Chemistry and coordination environments affect such properties. We have measured the hyperfine parameters of iron‐bearing rhyolitic and basaltic glasses up to ~120 GPa and ~100 GPa, respectively, in a neon pressure medium using time domain synchrotron Mössbauer spectroscopy. The spectra for rhyolitic and basaltic glasses are well explained by three high‐spin Fe 2+ ‐like sites with distinct quadrupole splittings. Absence of detectable ferric iron was confirmed with optical absorption spectroscopy. The sites with relatively high and intermediate quadrupole splittings are likely a result of fivefold and sixfold coordination environments of ferrous iron that transition to higher coordination with increasing pressure. The ferrous site with a relatively low quadrupole splitting and isomer shift at low pressures may be related to a fourfold or a second fivefold ferrous iron site, which transitions to higher coordination in basaltic glass, but likely remains in low coordination in rhyolitic glass. These results indicate that iron experiences changes in its coordination environment with increasing pressure without undergoing a high‐spin to low‐spin transition. We compare our results to the hyperfine parameters of silicate glasses of different compositions. With the assumption that coordination environments in silicate glasses may serve as a good indicator for those in a melt, this study suggests that ferrous iron in chemically complex silicate melts likely exists in a high‐spin state throughout most of Earth's mantle. Key Points: Iron‐bearing rhyolitic and basaltic glasses exhibit distinct trends in quadrupole splittings and isomer shifts with increasing pressure Quadrupole splitting is affected by the composition of the silicate glasses, particularly sensitive to (K2 O + Na2 O)/FeO and (CaO + MgO)/FeO Rhyolitic and basaltic glasses remain in the high‐spin state up to at least ~120 and 100 GPa, respectively