8-2. Division of Global Dynamics

　In the Global Tectonics Group, the aim is to reveal origins of various types of tectonics on the Earth's surface from the viewpoint of
the global dynamics of the Earth's interior. Tectonics is the surface expression of the mechanical work done in association with the
evacuation of the heat generated within the Earth's mantle. It is believed that the tectonics of the present Earth has a form of plate
tectonics. Plate tectonics is characterized by weak plate boundaries, and in this case, the form of the mantle convection with plates is
close to the convection of a fluid with uniform viscosity. However, the actual states of stresses within plates indicate that this is not
the case; in many cases, the convection style deviates from the uniform viscosity one. Active tectonics, such as the continental drifts
and back-arc spreading, is seen in such cases. This indicates that the deviation from plate tectonics is rather the cause of the various
active tectonics seen on the Earth's surface (Fig.1). The following themes are currently investigated: (1) Plate motions around the
Japanese islands, (2) stresses within plates and slabs, (3) driving forces of plates, (4) mechanisms of slab earthquakes, and (5) Archean
tectonics and evolution of environment.

ig.1. Three categories of tectonics (a) and (a') Slab pull force is dominant and balanced with the collision force.
 (b) Negative slab pull force is balanced with the mantle drag force beneath the oceanic plate. (c) Plate recycles in
    a smooth way along with the mantle convection, and normal island arc tectonism occurs in the upper plate.
                         Non-trivial tectonics occurs in the cases other than case (c).

In the Magma Research Group, various studies on magmas have been conducted based on field studies and high pressure experiments
(Fig.2). Temporal variations in magmatic composition of several active volcanoes have been studied to understand the mode of their
future eruptions. Studies on the genesis of primary magmas have been conducted to understand the thermal condition and the
chemical composition of upper mantle. Experimental studies on magmas at high pressures have also targeted to understand the
crystallization process of the magma ocean in the early stage of the Earth. As we recently found aqueous fluids behave like magma at
high pressure, aqueous fluids in mantle condition became our research targets. Our recent research themes are as follows. (1) Study
on Deccan flood basalts, (2) Experimental study on the genesis of continental flood basalt magma, (3) Experimental study of the
composition and the behavior of aqueous fluid in the mantle, (4) Temporal variations of magmatic compositions of Izu-Oshima, Asama,
and MIyake-jima volcanoes, (5) Role of water in the crystallization of island-arc magmas, (6) Petrological study of Miyakejima 2000

　In the Group Researching the Evolution of Terrestrial Materials, the aim is to reveal the evolution of the Earth through the study of
volcanism on a global scale and the role of volatile elements in the Earth's interior. Since the presence of volatile elements in the
mantle will affect the characteristics of mantle materials seriously, it is important to examine the present and the past states of
volatile elements in the Earth's interior, which has not well been studied. To reveal such points, chemical and isotope (noble gases,
10Be/9Be) compositions, radiometric (K-Ar, Ar-Ar, 14C) ages, and mineral compositions have been investigated for mantle-derived
rocks and minerals, in addition to extra-terrestrial materials (Fig.3). The following themes are being investigated at present: (1) Present
state of volatile elements in the Earth's interior, (2) Origin of volatile elements in kimbelites, (3) Relationship between a hotspot
volcano and a mantle plume, (4) Recycled materials in the Earth's interior and the chemical structure of the Earth, (5) Relationship
between plateau formation and mantle plumes and formation ages of plateaus, (6) Material transfer and chemical characteristics at a
spreading system, (7) Verification for the contribution of sediments to the arc magma under the Japanese Islands based on 10Be.

　In the Geochemistry Group, we investigate the geochemical evolution of the earth and material transfer including phenomena related
to volcanic activity. Our current main topics are 1) to put geochronological constraints on the time scale of volcanisms in subduction
zones, and 2) to reconstruct the course of the chemical evolution of a magma, during production, as well as the transportation, and
the eruption of magma by micro-analytical techniques to read geochemical records in volcanic products. We will tackle the first target
by uranium radioactivity disequilibrium method. It is considered that magmatism in subduction zones is triggered by addition of fluid
expelled from a subducting slab. The addition of uranium by the dehydration events forms radioactivity disequilibrium between 238U and
230Th. Taking advantage of the phenomenon, we can put constraints on the time scale of magma movements from magma formation to
eruption. We have established a procedure to analyze the radioactivity disequilibrium by using an multi-collector Inductively Coupled
Mass Spectrometer (MC-ICP-MS) installed in ERI (Fig.4). We just started to apply this method to Izu arc as a test field. For the second
target, we have established analytical protocols for trace elements by a Laser Ablation system coupled with an ICP-MS. We also
established a micro-drilling technique for Sr isotopic analysis in a plagioclase phenocryst. We applied the two techniques to plagioclase
samples extracted from volcanic products from Unzen. Apart from the two projects, we established analytical schemes for Li and Pb
isotopic analysis. Two elements will work as powerful tracers to investigate material transfer in subduction areas. In addition, we have
undertaken the developments of analytical procedures for Hf isotopic analysis to investigate genesis of Archean komatiites and of
Platinum Group Elements analysis to investigate core-mantle interaction.