Natalia V. Solomatova, Jennifer M. Jackson, Wolfgang Sturhahn, George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125
IMPMC_Museum National d'Histoire Naturelle-CNRS
ABSTRACTThe 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 ~90 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 Fe2+-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.
last revised 11 -Aug-2017