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   HACTO, the Research Group of the Physics of the Earth's Interior, aims to study
   the structure, dynamics, and evolution of the solid Earth through studying the
   physical and chemical properties of the constituents of the Earth's interior.
   On this page, we explain our research policies. 


   [1] Motivation

   [2] Approach

  1. Motivation
 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

 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
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, but
what 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 ?

  2. Approach
 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
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.

ultrasonic method is the most fundamental method used to determine the elastic
 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
, which are critical in the realization of high precision, ultrasonic resonance

 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 
 measurements of the mantle minerals under the P-T conditions of the deep

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.

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 conduct
simultaneous 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.