14:15 〜 14:30
[SGC46-09] Tectonic control on 3He/4He isotopic composition of OIB: an interpretation
キーワード：海洋島玄武岩、マントル、同位体
The isotopic variation of OIB is commonly represented by just four end-member components with extreme isotopic signatures, i.e., DMM, EM1, EM2 and HIMU. In addition to these four end-member components, some researchers have proposed the existence of components with intermediate isotopic compositions between the four end-member components, such as FOZO, C and PHEM mainly based on 3He/4He ratios of OIB and MORB. Namely, Hart et al. (1992) argued that FOZO is derived from the lower mantle, because OIB with FOZO-like isotopic compositions tends to have high 3He/4He ratios. Similarly, Hanan and Graham (1996) proposed component C based on 3He/4He ratios of MORB, i.e., C is situated in isotopic field where MORB arrays converge and 3He/4He ratios of OIB increase. Similarly, Farley et al. (1992) advocated a PHEM component that is characterized by high 3He/4He ratios, FOZO-like Pb isotopic compositions, and Sr-Nd isotopic ratios near that of the bulk Earth.
Although the Pb-Nd-Sr isotopic compositions of these intermediate components may be explained by simple mixing of the four end-member components, high 3He/4He ratios of intermediate reservoirs cannot be explained by mixing of the end-member components because all of end components are inferred to be derived from recycled crustal materials which necessary have low 3He/4He ratios. One explanation that can link a high-3He/4He component and recycled crustal material, whose Pb-Nd-Sr isotopic compositions match to the isotopic composition of intermediate components, is a model in which the lithospheric thickness beneath a hotspot is a major factor in determining the chemical composition of OIB yielded by melting of a heterogeneous source. In this presentation, we will examine effect of thickness of lithosphere beneath a hot spot that should control degree of melting of a heterogeneous magma source to produce Pb-Nd-Sr-He isotopic composition suitable for the intermediate mantle components.
Although the Pb-Nd-Sr isotopic compositions of these intermediate components may be explained by simple mixing of the four end-member components, high 3He/4He ratios of intermediate reservoirs cannot be explained by mixing of the end-member components because all of end components are inferred to be derived from recycled crustal materials which necessary have low 3He/4He ratios. One explanation that can link a high-3He/4He component and recycled crustal material, whose Pb-Nd-Sr isotopic compositions match to the isotopic composition of intermediate components, is a model in which the lithospheric thickness beneath a hotspot is a major factor in determining the chemical composition of OIB yielded by melting of a heterogeneous source. In this presentation, we will examine effect of thickness of lithosphere beneath a hot spot that should control degree of melting of a heterogeneous magma source to produce Pb-Nd-Sr-He isotopic composition suitable for the intermediate mantle components.