[S-IT41_28AM2] 海洋プレートの一生：誕生から解体，そして復活

[SIT41-08] 大西洋中央海嶺北緯36度レインボー地域における3次元地震波速度構造

*新井 隆太¹、ダン ロバート¹、キャナレス パブロ²、ソン ロバート² (1.ハワイ大学、2.ウッズホール海洋研究所)

キーワード：Slow-spreading ridge, Oceanic core complex, Rainbow hydrothermal field, Mantle alteration, Hydrothermal circulation, Seismic tomography

Oceanic lithosphere formed along slow-spreading mid-ocean ridges is structurally and compositionally heterogeneous due to spatial and temporal variations in tectonic extension, magmatic accretion, and mantle temperature and composition. While mid-ocean ridges with greater magma supply host a greater abundance of hydrothermal systems, the relative roles of magmatic input, heat advection and faulting in controlling ridge structures are still poorly understood. These are particularly important to understanding formation and evolution of oceanic core complexes where ultramafic-hosted lithologies are exhumed at the seafloor by long-lived detachment faulting. The MARINER (Mid-Atlantic Ridge INtegrated Experiments at Rainbow) seismic and geophysical mapping experiment was designed to examine the relationship between tectonic rifting, heat/melt supply, and oceanic core complex formation at a non-transform offset of the Mid-Atlantic Ridge, 36⁰14’N, the site of the ultramafic-hosted Rainbow hydrothermal system. Using the seismic refraction data from this experiment, we constructed three-dimensional tomographic images of the crust and upper mantle around the Rainbow area. The seismic velocity images reveal undulations in crustal thickness across the ultramafic Rainbow massif, indicating temporal variations in melt supply, magmatic processes, and crustal construction. Previous studies suggest that a current heat source for the vents, which probably arises from a magmatic body, is required just beneath the hydrothermal vent, but the tomography does not detect a low-velocity anomaly indicating a significant magmatic system or high-temperature region beneath the Rainbow vent site. The only candidate region for high-temperatures and perhaps melt at shallow levels is much further to the south, and located roughly beneath the central valley of the spreading center. At the Rainbow massif, where mantle rocks have been recovered by direct sampling, mantle velocities near the seafloor are significantly reduced to ∼5 km/s. This velocity reduction implies that an active hydrothermal circulation system altered the mantle via recharge and discharge of seawater.