海洋マントルカンラン岩から探る海洋リソスフェアの熱化学状態

Abstract

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 Earth’s 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.