Pictorial 3 Photomicrographs of Mantle Materials マントル物質を偏光顕微鏡で観察してみよう．岩石を 0.03 mm 程度の厚さの薄片にして偏光顕微鏡で観察する手法は固体 地球の研究の基本である．構成鉱物や組織がよくわかる．図 1 ～図 4 は直交ポーラー，図 5，図 6 は開放ポーラー．スケール は 1 mm(図 1 ～図 4)および 0.1 mm(図 5，図 6) ． Let us observe mantle materials in thin sections, 0.03 mm in thickness, under a microscope. This is a fundamental method for solid-earth scientists to understand textures and mineral compositions. Figs. 1 to 4 (scale bar, 1 mm) under crossed-polarized light, and Figs. 5 and 6 (scale bar, 0.1 mm) under plane-polarized light.

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Pictorial 2 Mantle Materials Found as Solid-intrusive Rocks 海嶺・断裂帯や沈み込みプレート境界沿いではマントル物質が貫入・定置している．蛇紋岩化などの低温の変質は免れない ものの，時として大規模な岩体をなし，最上部マントルの性質や地殻とマントルの関係などの観察に適している． Mantle-derived rocks are exposed along mid-ocean ridges and oceanic fracture zones, as well as past and current convergent plate boundaries. Although suffering from low-temperature alteration during and after emplacement, some occur as large complexes (e.g., ophiolites) , giving us information on the petrological characteristics of the upper mantle, as well as crust-mantle transition.

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Thermochemical state of the oceanic lithosphere constrained from oceanic mantle peridotites 秋澤 紀克 (Norikatsu AKIZAWA) ＊, ＊＊The life cycle of the oceanic lithosphere commences in the spreading axis and ends in the subduction zone. To trace the cooling and evolutional history of the Earth, the change in thermochemical state during the life cycle of present-day oceanic lithosphere is desired to be elucidated. In terms of the material science, spatial limitation of human-accessible Earth interior is a bottleneck in reconstructing the thermochemical state of the oceanic lithosphere. Yet, by combining active sampling methods using ocean research vessels (ocean drilling, ocean bottom dredging, submersible survey, etc.) and passive sampling methods using Earthʼs deep materials exposed to the surface owing to tectonic forces and volcanoes, we can collect samples that cover a considerable dimension. Here, I present efforts toward the elucidation of the thermochemical state of the oceanic lithosphere during its life cycle from the spreading axis to the subduction zone. The Oman ophiolite is presented as an analogue of oceanic lithosphere formed in the vicinity of a fast-spreading axis, whereas the peridotite xenoliths from Tahiti Island are treated as an analogue of thermochemically disturbed oceanic lithosphere by a mantle plume, and those from petit-spots are considered as an analogue less affected by thermochemical disturbance considering the lack of mantle plume beneath the petit-spots. A heterogeneous thermal state corresponding to the segment structure is inferred in the fast-spreading axis. The thermochemical state of the aging oceanic lithosphere is modified by mantle plume and petit-spot magmatism, but pristine state can be reconstructed by using suitable peridotite xenoliths whose heating-cooling and melting history is well characterized. The peridotite xenoliths from the petit-spots can enhance a step toward reconstructing the thermochemical state of the deep oceanic lithosphere because deep-rooted garnet-stable peridotite xenoliths can be found.