Geoscience Reference
In-Depth Information
8
Chemical and Physical Properties and
Thermal State of the Core
EIJI OHTANI
Department of Earth and Planetary Materials Science, Graduate School of Science,
Tohoku University, Sendai, Japan
8.1
Introduction
Abstract
Recent seismological studies revealed several
important characteristics of the Earth's central
region. There is a seismic velocity anomaly at
the core-mantle boundary, where an ultra-low
velocity zone has been reported from the seismo-
logical observations (e.g., Garnero & Helmberger,
1998). This anomaly may have originated from a
compositional anomaly such as iron enrichment
(e.g., Mao
et al
., 2006) formed by the reaction of
the mantle with the core or by a dense melt at
the base of the lower mantle (e.g., Ohtani, 1983;
Williams & Garnero, 1996; Stixrude & Karki,
2005; Labrosse
et al
., 2007). Some seismological
studies also have suggested a low P-wave velocity
anomaly at the top of the outer core, suggesting
some compositional anomalies at the outermost
part of the outer core (e.g., Tanaka, 2007; Helf-
frich & Kaneshima, 2010). Recent seismological
studies have revealed the asymmetric structure
of the inner core (Deuss
et al
., 2010). To explain
the physical properties of the core that have
been observed in seismology, we need more ex-
perimental data on phase relations and physical
properties of the candidate materials of the core.
The present composition and properties of the
core reflect formation processes of the Earth and
planets. Several light elements such as Si, O,
S, C and H have been proposed so far (e.g.,
Poirier, 1994). Formation at high temperatures
Recent advances in high-pressure mineral physics
of the Earth's central regions are reviewed. Several
new phase transitions have been reported in the
lower mantle such as the spin and post-perovskite
transitions. In addition, the existence of a dense
melt at the core-mantle boundary (CMB) is pro-
posed for the ultra-low velocity zone. The density
of the core is lower than that of pure iron un-
der the core conditions. Recent analyses of the
equation of state and the sound velocity of solid
and liquid iron and iron-nickel alloys strongly
suggest that the core contains light elements.
Possible candidates for the light elements in the
core are summarized. The nature and amounts of
light elements in the core are the key to clarifying
the early processes of formation of the Earth. The
phase relations, compression, and sound velocity
of iron-light-element alloys have been studied in-
tensively, based on the static experiments, shock
compression, and theoretical works. The present
status of these works and their implications for
the compositions of the outer and inner cores
are summarized. To achieve a definite conclu-
sion on the light element contents of the core,
we need to establish pressure scales at the high
pressure and temperature of the Earth's core, and
to measure the density and sound velocity of the
iron-light-element alloys covering the pressure
and temperature conditions of the inner core.
