Research for the Earth Using "Time" as a Weapon: Present Status of Dating and Clarifying the Chemical State of the Earth's Deep Interior Based on Isotope Ratios

Based on radiogenic isotopes, we can obtain information about the Earth related to time. Radiometric dating is a typical example. Due to the advanced development of current analytical techniques, radiometric ages covering the Earth's entire history can be obtained with an error of less than 1 ％ , even for a mineral crystal. However, there still remain problems to be clarified including the reliability of decay constants and the meanings of value obtained. In another approach, an isotope ratio including a radiogenic isotope can be used to clarify the evolution of the Earth. By applying multiple isotope systematics for typical volcanic rocks such as Mid-oceanic ridge basalts （ MORBs ） and oceanic island basalts （ OIBs ） , we conjecture the chemical state of the Earth's interior such as the degree of chemical fractionation and degassing. As an additional material used for clarifying the Earth's deep interior, I demonstrate the significance of kimberlites which might reflect the state of the Earth's deep interior more directly than OIBs.