Jennifer M. Jackson1*, Wolfgang Sturhahn2, Guoyin Shen3, Jiyong Zhao2, Michael Y. Hu4, Daniel Errandonea4, and Jay D. Bass1
1Department of Geology, University
of Illinois, Urbana, IL, U.S.A.
2Advanced Photon Source. Argonne National Laboratory, Argonne, IL,
U.S.A.
3Consortium for Advanced Radiation Sources, University of Chicago,
IL, U.S.A.
4HP-CAT and Carnegie Institution of Washington, Advanced Photon Source,
Argonne, IL, U.S.A.
*jmjackso@uiuc.edu
Knowledge of iron valancies in silicate perovskite is relevant to our understanding
of the physical and chemical properties of Earth’s lower mantle such as
transport properties, mechanical behavior, and element partitioning. Recent
studies have proposed that magnesium silicate perovskite is the principle sink
for ferric iron in Earth’s lower mantle [e.g. 1]. Previous investigations
on such perovskite samples have been performed at various temperatures, but
at room-pressure [e.g. 1]. In this study, we have measured the iron
valancies of two Fe-bearing silicate perovskite samples, Fe0.05Mg0.95SiO3
(Pv05) to 120 GPa and Fe0.1Mg0.9SiO3
(Pv10) to 75 GPa, at ambient temperature using diamond anvil cells.
Such investigations of extremely small and dilute 57Fe-bearing samples
have become possible after the development of synchrotron Mössbauer spectroscopy
(SMS), also known as nuclear forward scattering. The SMS data were taken at
beamline 3-ID of the Advanced Photon Source. The samples were synthesized in
a multi-anvil apparatus as described by Fei et al. [2], and the perovskite
structure for Pv05 and Pv10 was confirmed by X-ray diffraction.
Our results are explained in the framework of the “three-doublet”
model [2], which assumes two Fe2+ like sites and one Fe3+-like site
that are well distinguishable by the hyperfine fields at the location of the
Fe nuclei. At low pressures, the Fe3+/FePvtot is about
0.40 for both samples. Our results show that at pressures reaching into the
lower mantle the fraction of Fe3+ remains essentially unchanged.
The quadrupole splittings of all sites first increase with increasing pressure,
which suggests an increasingly distorted (non cubic) local iron environment.
Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05,
the quadrupole splittings are relatively constant suggesting an increasing resistance
of the lattice against further distortion. To our knowledge, these are the first
measurements of iron valancies in silicate perovskite under pressures characteristic
of Earth’s lower mantle.
[1] McCammon, C.A., Nature, 387, 694-696 (1997).
[2] Fei, Y., Virgo, D., Mysen, B. O., Wang, Y., and Mao, H.K., Am. Min., 79,
826-837 (1994).