Seismological Constraints on Core Composition from Fe-O-S Liquid Immiscibility
George Helffrich1* and
Satoshi Kaneshima2
Earth's core is composed primarily of iron (Fe) with about 10%
by weight of lighter elements. The lighter elements are progressively
enriched in the liquid outer core as the core cools and the
inner core crystallizes. Thermodynamic modeling of Fe-O-S liquids
shows that immiscible liquids can exist at outer-core pressures
(136 to 330 gigapascals) at temperatures below 5200 kelvin and
lead to layering in the outer core if the concentrations of
the lighter elements are high enough. We found no evidence for
layering in the outer core in the travel times and wave forms
of
P4
KP seismic waves that reflect internally in the core. The
absence of layers therefore constrains outer-core compositions
in the Fe-O-S system to be no richer than 6 ± 1 weight
% (wt %) O and 2 to 15 wt % S. A single core liquid composition
of 10.5 ± 3.5 wt % S and 1.5 ± 1.5 wt % O is compatible
with wave speeds and densities throughout the outer core.
2 Earth and Planetary Science, Tokyo Institute of Technology, 1-2-1 Ookayama, Meguro-ku, Tokyo 152-6551, Japan.
* To whom correspondence should be addressed. E-mail: george{at}geology.bristol.ac.uk
