マントルの進化 : Nd同位体比による検討

概要

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Sm/Nd system is more suitable than Rb/Sr system to clarify the formation process of upper mantle. We have collected many Nd isotopic data for intermediate, basic and ultra-basic igneous rocks from many parts of the world. These data have been re-calculated as ε_<Nd> value (DEPAOLO & WASSERBURG, 1976a) using bulk earth's parameters as follows; ^<143>Nd/^<144>Nd (present ratio)=0.512640, ^<147>Sm/^<144>Nd=:0.1967, λ(^<147>Sm)=6.54×10^<-12>y^<-1>. These ε_<Nd> values have been plotted in Fig. 2. Fig. 1 is shown more precisely than Fig. 2. We have examined this figure and field relationships for rocks quoted in this figure in detail, and furthermore, examined the ε_<Nd> values for rocks as follows; intermediate and basic igneous rocks being younger than late Cretaceous (Fig. 3), acid igneous rocks (Fig. 4), kimberlites, alkali basalts, mantle origin nodules (Fig. 7). As the result, two models for the formation process of upper mantle are proposed, which are shown in Fig. 5 (Model-1) and Fig. 6 (Model-2). These models are reasonable in normal region, but unreasonable in Dupal anomaly (HART, 1984) and its surrounding regions. The difference between Model-1 and Model-2 is mainly due to that of the formation process of oceanic lithosphere. For determination of real model, it is necessary to study the ophiolites which are older than l.3 Ga.
地学団体研究会の論文
1987-01-25