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

Abstract

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 degas-sing. 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. Key words： time, radiogenic isotope, dating, isotope ratio, Earth's deep interior, chemical state, kimberlite ：時間放射性起源同位体年代測定同位体比地球深部化学的環境キンバーライト ＊ 独立行政法人海洋研究開発機構高知コア研究所（ ） ＊ Kochi Institute for Core Sample Research, JAMSTEC 本稿は 2008 年 2 月 20 日に行われた地学クラブ講演会の講演報告である． 668-