6-2）Division of Global Dynamics

In this division, we are investigating the mechanism of phenomena related to earthquakes and volcanism on a global scale based on theoretical, observational, and experimental studies, including data analyses and laboratory work.

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, (5) mechanisms of tsunami earthquakes, and (6) mechanisms of interplate earthquakes.

Fig.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 Dynamics Research Group, mantle dynamics is studied from the viewpoint of fluid dynamics. The water dehydrated from the subducting slab may produce a low-viscosity zone under the back-arc. The existence of low-viscosity zone coupled with the shear flow induced by the subducting slab may trigger a small-scale roll-like convection, whose axes are perpendicular to the subduction zone (Fig.2). We tested this hypothesis using 3-D modeling of mantle flow and found that such a small-scale convection may be an origin of "hot fingers", which was recently proposed to explain the distribution of volcanoes, Bouger anomaly and seismic tomography in north east Japan.

 

Fig.2.  Isothermal surface caused by the small-scale convection beneath the backarc.

In the Magma Research Group, various studies on magmas have been conducted based on field studies and high pressure experiments (Fig.3). 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) Experimental study on the genesis of continental flood basalt magma, (2) Experimental study of the composition and the behavior of aqueous fluid in the mantle, (3) Temporal variations of magmatic compositions of Fuji, Izu-Oshima, and MIyake-jima volcanoes, (4) Role of water in the crystallization of island-arc magmas, (5) Petrological study of Miyakejima 2000 eruption, (6) Magmatological study of explosive volcanic eruptions of Mt Fuji.

 

Fig.3. Internally heated pressure vessel. It can generate pressure up to 0.5GPa and temperature up to 1,500℃. Composition and texture of pre-eruptive magma are studied.

 

In the Group Researching the Evolution of Terrestrial Materials, the aims are to reveal the evolution of the Earth through the study of volcanism on a global scale and to clarify the chemical conditions in the Earth's deep interior incuding the states of volatile elements. 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.4). At present, the main research theme is to reveal the chemical conditions of the Earth's deep interior based on chemical and isotope studies of kimberlites, rocks derived from plume areas and mantle xenoliths. Furthermore, it is also an important theme to study the material recycling in the Earth's interior by comparing the chemical and isotope signatures among those of volcanic rocks from various areas such as continents, arcs and ridges.

 

Fig.4.  Noble gas mass spectrometer (VG5400).

 

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 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 are tackling the first target by uranium radioactivity disequilibrium method. We have established analytical methods for ²³⁸U-²³⁰Th-²²⁶Ra with a multi-collector inductively Coupled Plasma Mass Spectrometer in ERI. It is considered that magmatisms in subduction zones are triggered by addition of fluid expelled from a subducting slab. The addition of uranium and radium by the dehydration events forms radioactivity disequilibrium between ²³⁸U-²³⁰Th and ²³⁰Th-²²⁶Ra. Taking advantage of the phenomenon, we can put constraints on the time scale of magma movements from magma formation to eruption. We started to apply this method to volcanic rocks from Izu arc. The results indicate that the volcanic rocks from Izu arc are enriched in ²³⁸U and ²²⁶Ra relative to ²³⁰Th like those from other island arc systems. If the radioactivity disequilibrium between ²³⁰Th-²²⁶Ra formed during the dehydration of the slab, the results indicate that the magmas in this area erupt within a few thousand years after their formations. The degree of radioactivity disequilibrium between ²³⁸U and ²³⁰Th decreases from Izu-Oshima, Miyakejima to Niijima, as the depth of the slab increases. The lateral variation across the arc seems to suggest that the addition of the slab-derived components decreases as the depth of the slab increases. However no lateral variation in the degree of the radioactivity disequilibrium between 230Th and 226Ra was observed. We are planning to confirm if the radioactive disequilibrium between thorium and radium was the results of the fluid movements from the slab by investigating relations between the degree of the radioactivity disequilibrium and the other geochemical tracers, such as ¹⁰Be.

 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（Fig.5）.  We have revealed that the plagioclase in the last three eruptions of the volcano have common origin. Apart from the two projects, we established analytical schemes for Li and Hf isotopic analysis. Two elements will work as powerful tracers to investigate material transfer in subduction areas. In addition, we worked on Hf isotopic study on Archean komatiites and of Platinum Group Elements analysis to investigate core-mantle interaction.

 

Fig.5.  (a) a microscopic image of a plagioclase (950518-pl.2) extracted from a lava erupted in 1995 from Unzen volcano, (b), (c) and (d) show variations of Anorthite value, La/Nd and Sr/Ba abundance ratios, and Sr isotopic ratios in the phenocryst, respectively. Remnants of mineral desorption and layers of inclusions were shown as hatched areas in (b).  Squares with cross inside and filled circles denotes La/Nd and Sr/Ba ratios in (c), respectively.  The precision of the ratios were about 15%.  Rectangles in (d) denotes Sr isotopic ratios.  Their vertical width denotes error of isotopic analyses, while horizontal width denotes uncertainty in a drilling position.