マントルの同位体的多様性

マントル進化へのアプローチ

抄録

The isotopic descriptions of present-day mantle derived rocks reveal the regional and global heterogeneities of the Earth's mantle with systematic variations. The mantle heterogeneities and systematic variations include Dupal anomaly, mantle array, mantle plane, and LoNd array. They led to the hypotheses involving the mixing of some mantle components. The mantle components are identified as follows ; DMM (most depleted component in the Earth), EM II (enriched component derived from sediments and subducted oceanic crust), EM I (end-member of LoNd array), HIMU (end-member of LoNd array with high U/Pb ratio), and PREMA (common and major components in the Earth's mantle, synonymous with, PUM, CHUR, UR and BSE). These mantle components could be generated by the principal processes of chemical separation, time effect and component mixing. The chemical separation process changes the parent/daughter ratios, the time effect process varies the isotope ratios, and the component mixing process between mantle components resulted in isotopic variations of the magmas derived from mantles.
In the Earth's mantle evolution, two important global events occurred before about 3.8 Ga (1st event) and after 2 Ga (2nd event). The 1st event might be related to stratification of the early Earth, such as, the separation of the mantle and core, and/or the chemical stratification of mantle involving magma ocean. For a period of 3.8^-2 Ga, the chemical structure of the mantle had been preserved. The second global event started at 2 Ga. This event might be related to the formation of the solid inner core from primitive liquid core, which might activate the mantle convection. The convection which mixed the mantle components should make the present isotopic variations of the mantle.