Significance of He-Pb isotope systematics in Earth Sciences

Significance of He-Pb isotope systematics in Earth Sciences is discussed by showing some examples, which has not well been developed so far in spite of its usefulness. Both 4He and lead isotopes (206Pb, 207Pb 208Pb) are decay products from U and Th, and Th/U ratios in the Earth's interior can be regarded to be varied in a relatively narrow range (2-4). Hence, the production of 4He and 206-208Pb can be approximated to be a function of U-content in the system. However, this approximation does not change the shape of the growth curve in the 4He-206-208Pb diagrams significantly. In such diagrams, the relative difference in the mobility of He and Pb during the formation of the system or later loss from the system is the main factor to control the variation of the growth curve. Hence, such a diagram can be used as a sensitive tool to trace the processes which might occur in the Earth's interior. (author)