講演情報
[SGC46-03]The numerical approach to reveal the structure of the large-scale deep mantle, focusing on subduction slabs by major element and isotopic compositions of OIBs
*宮嶋 郁佳1、平野 直人1 (1.東北大学)
キーワード：
マントル構造、llsvp、海洋地殻、沈み込み
The mantle structure has been observed for many years using seismic tomography revealed a heterogeneous structure. Among these heterogeneous features, the Large Low Shear Velocity Province, LLSVP, distributed at the core-mantle boundary. This region exhibits a negative S-wave velocity anomaly at the core-mantle boundary, subdivided into two distinct regions: beneath Pacific Ocean and African Continent. It is also recognized that several mantle plumes originate from this region. Although the origin of this heterogeneous structure remains unclear, one of the candidates is subducted oceanic crust piled on core-mantle boundary. In this study, the effect of the subduction of oceanic crust to source magma of OIB is estimated.
OIB compositional data was filtered by MgO > 8.0 wt% and FeOt > 8.8wt%. And the data was divided into two by the location, Pacific hotspots and Atlantic and Indian hotspots. The number of the data on Pacific hotspots is 4,757 and that on Atlantic and Indian hotspots is 4,919. Under the assumption that OIB's source mantle is composed of two types of mantle; KLB-1 lherzolite (Lz) assumed as primitive mantle and KG1 silica-undersaturated pyroxenite (Px) assumed as subducted oceanic crust, Px ratio in source mantle of each rock were estimated by FeOt concentration. OIB is thought to be composed of Lz derived melt and Px derived melt, and the formation of OIB is assumed as mantle is melting at the bottom of lithosphere. The mantle melting is calculated by pymelt Python module (Matthews et al., 2020). Our result shows that Px ratio is up to approximately 10 % on Atlantic and Indian hotspots in contrast to almost zero on Pacific hotspots. Considering Sr-Nd-Pb isotope composition, the estimated Px ratio is reasonable to evaluate subduction component in source mantle. Based on this, the impact of subducted oceanic crust is higher beneath African Continent rather than beneath Pacific Ocean.
Furthermore, the Sr-Nd-Pb isotopic data excluded the influence of subduction component based on this estimation shows that data beneath African Continent is FOZO-like component and those beneath Pacific Ocean is more depleted component. Most of the source magma of Atlantic and Indian hotspots is illustrated by addiction of EM1, 2, HIMU endmembers into FOZO with no contradiction between Px ratio and Sr-Nd-Pb isotope composition. Some of the source magma of Atlantic and Indian hotspots and most of Pacific hotspots is illustrated by addiction of FOZO into DMM with no contradiction between Px ratio and Sr-Nd-Pb isotope composition.
OIB compositional data was filtered by MgO > 8.0 wt% and FeOt > 8.8wt%. And the data was divided into two by the location, Pacific hotspots and Atlantic and Indian hotspots. The number of the data on Pacific hotspots is 4,757 and that on Atlantic and Indian hotspots is 4,919. Under the assumption that OIB's source mantle is composed of two types of mantle; KLB-1 lherzolite (Lz) assumed as primitive mantle and KG1 silica-undersaturated pyroxenite (Px) assumed as subducted oceanic crust, Px ratio in source mantle of each rock were estimated by FeOt concentration. OIB is thought to be composed of Lz derived melt and Px derived melt, and the formation of OIB is assumed as mantle is melting at the bottom of lithosphere. The mantle melting is calculated by pymelt Python module (Matthews et al., 2020). Our result shows that Px ratio is up to approximately 10 % on Atlantic and Indian hotspots in contrast to almost zero on Pacific hotspots. Considering Sr-Nd-Pb isotope composition, the estimated Px ratio is reasonable to evaluate subduction component in source mantle. Based on this, the impact of subducted oceanic crust is higher beneath African Continent rather than beneath Pacific Ocean.
Furthermore, the Sr-Nd-Pb isotopic data excluded the influence of subduction component based on this estimation shows that data beneath African Continent is FOZO-like component and those beneath Pacific Ocean is more depleted component. Most of the source magma of Atlantic and Indian hotspots is illustrated by addiction of EM1, 2, HIMU endmembers into FOZO with no contradiction between Px ratio and Sr-Nd-Pb isotope composition. Some of the source magma of Atlantic and Indian hotspots and most of Pacific hotspots is illustrated by addiction of FOZO into DMM with no contradiction between Px ratio and Sr-Nd-Pb isotope composition.