Electronic environments of
ferrous iron in
rhyolitic and basaltic glasses at high pressure
Natalia V. Solomatova, Jennifer M. Jackson, Wolfgang Sturhahn, George
R. Rossman
Division of
Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125
Mathieu Roskosz
IMPMC_Museum National
d'Histoire Naturelle-CNRS
Paris, France
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 ~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.