Geochemical and environmental-chemical studies based on speciation mainly using X-ray absorption fine structure (XAFS) spectroscopy by the author have been reviewed in this manuscript. These studies revealed that we could understand physico-chemical processes behind the distribution and isotopic data obtained in various geochemical studies by this approach, which can be called as molecular geochemistry. This approach allows us to interpret geochemical cycles of various elements from atomic and molecular scale levels, which in turn enables us (i) to extract more information from geochemical data related to earth history and (ii) to predict environment in future more accurately. The studies introduced here include environmental chemistry of rare earth elements and actinides, behavior of toxic elements at earth surface, environmental impacts of speciation of various elements in aerosols, enrichment of various elements to ferromanganese oxides and its application to paleo-environment studies, understanding of isotope fractionation based on speciation data, and migration of radioactive nuclides emitted by the Fukushima Dai-ichi Nuclear Power Plant accident. It was also shown that development of X-ray spectroscopic methods applied to geochemistry and environmental chemistry has opened new fields in environmental geochemistry and trace element geochemistry. Through these various topics, I would like to emphasize that systematic studies on behavior of various elements in environment in terms of physico-chemical viewpoint can provide various new ideas in wide fields in geochemistry as was suggested by Prof. V. M. Goldschmidt. The basic knowledge of various elements can be a firm basis to use geochemistry to obtain more general information in earth and environmental sciences. The attractiveness and importance of molecular geochemistry indicated in this manuscript suggests that this field can be one of drivers to develop new geochemistry in 21st century.

Earth's mantle formed at 4.6 Ga and evolved through the Hadean magma ocean stage and subsequent plate tectonics stage since 3.5 Ga. Mantle convection driven by the internal heat is the major driving force of the plate tectonics with considerable tectonic roles of both oceanic and continental plates formed at mid-ocean ridges (MOR) and subduction zones (SZ), respectively. The MOR and SZ regions are the places of plate formations by intensive magma geneses where significant element fractionations between solids and melts are taking place. The MOR and SZ regions are the major factories of tectonic and geochemical mantle evolutions because their products of plates and residual mantles are mixed back into the mantle by stirring or isolated almost permanently. The geochemical fractionations in the MOR and SZ magmatism are modelled based on petrochemical mass balance and elemental and isotopic growths of the magmas and the residues are examined. These combined to enable depicting the thermal, chemical, and isotopic evolutions of the Earth's mantle over 3.5 Gyr. The present-day mantle appears to be geochemically heterogeneous and forms large mantle domains in both deep and shallow portions by Mesoproterozoic (1.7 Ga). These suggest relatively sluggish mantle convection after Mesoproterozoic due to mantle cooling.

In assessing the safety of geological disposal of high-level radioactive waste, groundwater chemistry is an important factor influencing the performance of engineered materials and the behavior of radionuclides. It is essential to understand the hydrochemical changing process and stable condition after the closure of the disposal facilities. In this study, a simulated experimental drift was constructed in granite at a depth of 500 m at the Mizunami Underground Research Laboratory, and the hydrochemical condition and related processes after drift closure were observed. The groundwater flow and chemistry around the excavated drift changed owing to water inflow along fractures into the drift. The redox potential increased owing to the infiltration of oxygen from the drift. After closing the drift, the groundwater evolved chemically with time owing to mixing of the groundwaters derived from surrounding rocks, dissolution of shotcrete, and microbial reactions. The redox potential plunged to the initial baseline value in three months owing to the reducing activity of microbes, while the groundwater became alkaline owing to the influence of the dissolved shotcrete. If the content of a pH-buffer mineral in a rock is small compared with the cementaceous materials used, the alkaline groundwater probably spreads broadly in surrounding rock. In the future, it is necessary to verify the water chemistry evolved by the interaction between the alkaline groundwater and granite minerals.