[SIT26-15] Is there primordial reservoir in lower mantle?
キーワード：核ーマントル境界、タングステン同位体、拡散、浸透
Is there primordial reservoir in lower mantle?
Takashi Yoshino (Institute for Planetary Materials, Okayama University)
Tungsten isotope composition provide important constraints on the sources of the ocean-island basalts (OIB). If the core formation occurred within the first 60 Myr of the solar system formation, the silicate mantle should be characterized by high Hf/W and positive µ182W, whereas the metallic core has very low Hf/W and negative µ182W. Recently µ182W of the young OIBs with high 3He/4He have shown two distinct features: positive µ182W from Phanerozoic flood basalts (Ontong Java and Baffin Bay) indicating a presence of primordial reservoir [1], and negative µ182W from modern OIBs (Hawaii, Somoa, Iceland) [2]. However, later study suggested that a positive µ182W anomaly can potentially be caused by nuclear shift isotope fractionation affecting primarily the odd isotope (183W) and basalts from Ontong Java have also shown negative µ182W anomaly [3]. Thus, there is no evidence from tungsten isotopes to constrain the existence of a primordial reservoir in the lower mantle. On the other hand, negative µ182W anomaly becomes a universal feature of the modern OIBs. One possibility to produce the negative µ182W is chemical interaction of the mantle with the Earth’s outer core. An experimental study has shown that W grain boundary diffusion in the lower mantle phases is quite fast process and displays strong temperature dependence [4]. W isotope can be modified significantly at the base of the lower mantle through the whole Earth’s history. When highly-oxidized slabs accumulate at the CMB oxidizing the outer core at the interface, a large W flux with negative µ182W can be added to the silicate mantle. As a result, the source region of the OIB would be effectively modified to a negative µ182W. A good correlation of µ182W and 3He/4He observed in modern OIBs invokes that 3He is also provided by the outer core liquid. In this case, there is no necessity to envision a primordial reservoir in the deep lower mantle. Future studies should be accompanied by coupled investigations with diffusion of 3He/4He to further constrain core-mantle interaction.
References
[1] H. Rizo et al., 2016. Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Science 352, 809–812.
[2] A. Mundl et al., 2017. Tungsten-182 heterogeneity in modern ocean island basalts. Science 356, 66-69.
[3] T.S. Kruijer, T. Kleine, T., 2018. No 182W excess in the Ontong Java source. Chem. Geol. 485, 24-31.
[4] T. Yoshino et al., 2020. Grain boundary diffusion of W in lower mantle phase with implications for isotopic heterogeneity in oceanic island basalts by core-mantle interactions. Earth Plane. Sci. Lett., 530, 115887
Takashi Yoshino (Institute for Planetary Materials, Okayama University)
Tungsten isotope composition provide important constraints on the sources of the ocean-island basalts (OIB). If the core formation occurred within the first 60 Myr of the solar system formation, the silicate mantle should be characterized by high Hf/W and positive µ182W, whereas the metallic core has very low Hf/W and negative µ182W. Recently µ182W of the young OIBs with high 3He/4He have shown two distinct features: positive µ182W from Phanerozoic flood basalts (Ontong Java and Baffin Bay) indicating a presence of primordial reservoir [1], and negative µ182W from modern OIBs (Hawaii, Somoa, Iceland) [2]. However, later study suggested that a positive µ182W anomaly can potentially be caused by nuclear shift isotope fractionation affecting primarily the odd isotope (183W) and basalts from Ontong Java have also shown negative µ182W anomaly [3]. Thus, there is no evidence from tungsten isotopes to constrain the existence of a primordial reservoir in the lower mantle. On the other hand, negative µ182W anomaly becomes a universal feature of the modern OIBs. One possibility to produce the negative µ182W is chemical interaction of the mantle with the Earth’s outer core. An experimental study has shown that W grain boundary diffusion in the lower mantle phases is quite fast process and displays strong temperature dependence [4]. W isotope can be modified significantly at the base of the lower mantle through the whole Earth’s history. When highly-oxidized slabs accumulate at the CMB oxidizing the outer core at the interface, a large W flux with negative µ182W can be added to the silicate mantle. As a result, the source region of the OIB would be effectively modified to a negative µ182W. A good correlation of µ182W and 3He/4He observed in modern OIBs invokes that 3He is also provided by the outer core liquid. In this case, there is no necessity to envision a primordial reservoir in the deep lower mantle. Future studies should be accompanied by coupled investigations with diffusion of 3He/4He to further constrain core-mantle interaction.
References
[1] H. Rizo et al., 2016. Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Science 352, 809–812.
[2] A. Mundl et al., 2017. Tungsten-182 heterogeneity in modern ocean island basalts. Science 356, 66-69.
[3] T.S. Kruijer, T. Kleine, T., 2018. No 182W excess in the Ontong Java source. Chem. Geol. 485, 24-31.
[4] T. Yoshino et al., 2020. Grain boundary diffusion of W in lower mantle phase with implications for isotopic heterogeneity in oceanic island basalts by core-mantle interactions. Earth Plane. Sci. Lett., 530, 115887