Our motivation for research stems from the big, as yet unsolved problems
regarding the
Earth's interior, including:
1. What processes were dominant during the core-mantle segregation of the early Earth
stage?
It is believed that the Earth was made from carbonaceous chondritic meteorite
and that
core-mantle segregation was caused by the melting and migration of metallic iron.
In this stage, was the mantle a solid (rock) or liquid (such as a magma
ocean)?
2. What is the dominant flow in the mantle?
Although the mantle is mostly composed of solid rock, in the geological
time scale the mantle
flows like a liquid. This flow is referred to as 'mantle convection." Here, we pose the following
questions. What is the mechanism behind mantle convection?
How soft is the mantle? How fast is the convection rate?
3. How hot is the Earth's interior ?
The temperature of the Earth's interior increases with increasing depth according to boring
core data. It is clear that the Earth's interior is hot, butwhat is the precise temperature?
1,000°C? 10,000°C?
It is impossible to insert a thermometer directly deep into the Earth.
What is the best method for measuring temperature in the Earth's interior?
4. What constitutes the rock of the mantle?
The mantle is mainly composed of rock. Seismological observation has revealed
that
the mantle is heterogeneous in various characteristic regions.
For example, in the shallow lower mantle,
there are many regions where seismic waves are strongly reflected.
What does this indicate?
It is also thought that the materials in the bottom of the lower mantle
(with thickness of 200 km
above the core-mantle boundary, corresponding to ~123 GPa) differ from those in the rest of
the lower mantle based on observations of large seismic anomalies
(discontinuity and anisotropy).
This region is called the "D" layer." What are these anomalies derived from? Recent studies
have suggested that post-perovskite, which is a higher pressure phase transformed from
perovskite, plays a key role in the D" layer. Is this true?
5. What kinds of materials compose the Earth's core?
The Earth is covered by crust, and there is a mantle beneath it. Although it is not easy to
directly gather mantle material, it is possible to obtain materials of
the uppermost mantle.
However, it is still impossible to directly sample the core materials.
Although it is believed that the Earth's core is composed of iron-nickel alloy,
we do not know its precise chemical composition.
6. Is there a substantial amount of water in the Earth's interior?
We can observe a structure of the Earth's interior using methods of seismology and
electromagnetic study. We can find some problems not to be explained by
the physical
properties of water free rocks dry. For example, electrical conductivity
at the top of the
asthenosphere is much higher than that of peridotite we expected. Why
? Many researchers
consider this phenomenon by a presence of water in the mantle. If we assumed
to be a
presence of water in the Earth's interior, some problems can be solved.
But actual water content of MORB (Mid Ocean Ridge Basalt) is too small.
Is there really substantial amount of water in the Earth's interior ?
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Based on the above motivations for research, we attempt to understand more accurately the
Earth's interior. To this end, we are in the process of determining the
various physical
properties of the mantle constituent materials and comparing our data
with the results of geophysical
observations. Because the Earth's interior exists under high temperature and high pressure,
we must conduct experiments under similar extreme conditions in order
to
accurately determine the physical properties of the Earth's interior.
Our approaches are as follows:
1. Melting experiments of primordial materials under ultra high pressure
We create a "magma ocean" in our lab by melting peridotite,
of which the uppermost mantle is
thought to be composed, or chondrite, considered to be the most important
material
of the early Earth, under ultra high pressure conditions.
We simulate the segregation process of the metallic
core from the undifferentiated Earth based on studies of element partitioning between metal
and silicate melt and wetting properties.
2. Determination of the elastic properties of the materials in the Earth's interior
The most useful way to investigate the Earth's interior is through the
seismological approach.
Using this approach, we determine the elastic properties of minerals and
obtain information
on the chemical composition and temperature of the Earth's interior by
comparisons of
seismological data.
The ultrasonic method is the most fundamental method used to determine the elastic
properties.
We can determine the elastic constants of minerals by the ultrasonic method
up to 10GPa
(depth 100km). Further, we are in the process of developing new methods
of data analysys
and conducting more accurate measurement.
It is broadly known that Brillouin scattering spectroscopy in diamond anvil cells is currently
the only way to determine the elastic constants under the pressure conditions
at the lowermost mantle.
We are now developing this technique to investigate the elastic properties
of the
constituent minerals in the lower mantle under the relevant P-T conditions
in addition to our
conventional techniques for phase identification and pressure determination
using
the combined techniques of laser heating and X-ray diffraction measurements.
Ultrasonic resonance measurement is also known as one of the most reliable methods for
obtaining the elastic constants of minerals. Although it is difficult
to measure
at high pressures using this technique, it nonetheless enables us to get
highly accurate/precise data of the elastic constants at ambient pressure.
We are also developing a method to synthesize 'giant' single
crystals, which are critical in the realization of high precision, ultrasonic resonance
measurements.
We also conduct high resolution, inelastic X-ray scattering measurements
at our a synchrotron facility (SPring-8), which often allows us to measure
the elastic constants of materials that we can not measure by Brillouin
scattering or
ultrasonic resonance. We can therefore use these
techniques in a complementary manner for various materials.
By accumulating the data of each mantle phase using the new techniques mentioned above,
we can investigate the elasticity of the mantle mineral phases.
Then, we can reconstruct a mineralogical model of the deep mantle, which is multi-phase.
3. Determination of the electrical properties of mantle minerals
Electromagnetic observations provide us with important information such
as
the presence of water, magma, and iron content in the deep Earth.
From this point of view, we conduct electrical
conductivity measurements of the mantle minerals under the P-T conditions of the deep
mantle.
4. Rheology of mantle minerals
Diffusion creep is one of the flow mechanisms of the mantle materials.
The diffusion creep rate can be estimated by the self-diffusion coefficient
of elements
such as Si that compose the
framework of such minerals. We thus can estimate the flow low of the mantle
minerals from
the measurement of the Si self-diffusion coefficient using synthesized 'giant' single crystals.
The flow of multi-phase 'rock' does not only depend on its chemical composition, temperature,
and pressure, but also strongly on the spatial distribution and grain
size of each phase. We
therefore try to construct the flow on the 'bulk Earth' by the precise
analysis of the effect of
each parameter.
5. Equation of the state of mantle minerals
In order to resolve the structure and dynamics of the mantle, it is essential
to understand the
P-V-T relationships of the mantle minerals. Thermal expansion is an especially important
parameter in the estimation of temperature gradient/distribution in the Earth. We attempt to
determine the thermal expansion of mantle minerals by X-ray diffraction measurements
at high P-T conditions.
6. Phase equilibria
Phase determination of substances is one of the most basic problems in the study of
high-pressure materials. In-situ X-ray observation is our principal method of determining
equilibrant phases.
7. Heat transfer properties of minerals
The temperature distribution of the Earth's interior causes the buoyancy
considered to be
one of the most important driving forces of core and mantle dynamics.
Therefore, knowledge of the heat
transfer properties, such as thermal conductivity, thermal diffusivity, etc., of Earth materials is
essential for the simulation of temperature distribution.
We conductsimultaneous measurements of the thermal conductivity and thermal diffusivity of
minerals under high-pressure and high-temperature conditions.
8. Physico-chemical properties of fluid phases
The Earth's mantle is considered to exist mainly in a solid state.
Fluid phases have properties like high mobility, high diffusivity, high
electric conductivity,
and low elasticity, etc.
These phases consequently influence the properties of the Earth.
We measure the contact angles between
minerals and fluids and discuss the possible influences of fluids on the properties of rocks.
We also study the structure and dynamics of fluid phases using state-of-the-art techniques
such as X-ray Raman scattering.
9. Technical development for ultra high-pressure generation
It is a challenge to measure the physico-chemical properties of materials
under extreme
conditions of high-pressure and high-temperature corresponding to the Earth's deep interior.
We focus on the technical development of ultra high-pressure generationusing
sintered diamond anvils.
Conventional multi-anvil, high-pressure apparatuses can generate at most 30 GPa
corresponding to the pressure at 820 km depth. Although diamond anvil
cells are usually
used to generate higher pressures, they are not suitable for some measurements.
Therefore, the expansion of the pressure-temperature region accessible
with multi-anvil
apparatuses is desirable. We attempt to substantially raise the maximum
pressure
generatable by means of multi-anvil apparatuses by using sintered diamond as the second
stage anvils. We have established the current world record of 72 GPa,
corresponding to 1730 km depth.
This technical development was achieved not only through trial and error but also based on
the results of thorough stress analysis trials inside pressure cells.
These technical developments are also important in aspects other than pressure generation
using sintered diamonds. To this end, we constantly address the development of new
experimental techniques.
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