15:45 〜 16:00
[SCG59-08] Chemical and thermal state of oceanic lithosphere: reconstruction from petit-spot mantle xenoliths in the northwestern Pacific
キーワード：東北沖、海洋マントル、ザクロ石カンラン岩、海洋リソスフェア
The oceanic lithosphere is formed in the mid-ocean ridges and cools as it spreads away to the subduction zones. In the context of Earth’s material cycle, quantitative chemical and thermal state of the oceanic lithosphere is desired to be elucidated. As a step toward, we studied mantle xenoliths from petti-spots in the northwestern Pacific, where no seismic anomaly is imaged. The petit-spot-borne mantle xenoliths provide us unique chemical and thermal records avoiding modifications derived from the mantle plumes.
The petit-spot mantle xenoliths studied herein were collected at petit-spot Sites A and B in the northwestern Pacific using deep-submergence vehicle Shinkai 6500 during expeditions of YK05-06, YK20-14S, YK21-07S, and YK24-10S. They are small in size ranging from 1 to 5 cm in diameter, except for a big lherzolite with 15 cm-long diameters. The mantle xenoliths are classified as dunite, harzburgite, and lherzolite based on modal proportion of constituent minerals. The peridotites show variation in terms of the presence of spinel and garnet, and degree of melt depletion. Some of the peridotites include fine-grained mineral aggregates, which are broken-down products after pyrope-rich garnets considering their average bulk chemical compositions. Geothermobarometric pressure-temperature estimation results indicate that the peridotite xenoliths were derived from down to 3 GPa, where asthenosphere/lithosphere boundary is expected based on the geophysical investigations.
Depleted dunite/harzburgite layer is perched atop the reconstructed geochemical stratigraphy, which can be interpreted as basaltic melt extraction in the mid-ocean ridge. In contrast, the depleted harzburgites are emplaced at depths down to 3 GPa, which cannot be explained by “simple” depletion in the mid-ocean ridge. Because the petit-spot peridotite xenoliths cover a long range of the oceanic stratigraphy deep down to the lithosphere/asthenosphere boundary, we present more detailed chemical and thermal state of the whole oceanic lithosphere in the presentation.
The petit-spot mantle xenoliths studied herein were collected at petit-spot Sites A and B in the northwestern Pacific using deep-submergence vehicle Shinkai 6500 during expeditions of YK05-06, YK20-14S, YK21-07S, and YK24-10S. They are small in size ranging from 1 to 5 cm in diameter, except for a big lherzolite with 15 cm-long diameters. The mantle xenoliths are classified as dunite, harzburgite, and lherzolite based on modal proportion of constituent minerals. The peridotites show variation in terms of the presence of spinel and garnet, and degree of melt depletion. Some of the peridotites include fine-grained mineral aggregates, which are broken-down products after pyrope-rich garnets considering their average bulk chemical compositions. Geothermobarometric pressure-temperature estimation results indicate that the peridotite xenoliths were derived from down to 3 GPa, where asthenosphere/lithosphere boundary is expected based on the geophysical investigations.
Depleted dunite/harzburgite layer is perched atop the reconstructed geochemical stratigraphy, which can be interpreted as basaltic melt extraction in the mid-ocean ridge. In contrast, the depleted harzburgites are emplaced at depths down to 3 GPa, which cannot be explained by “simple” depletion in the mid-ocean ridge. Because the petit-spot peridotite xenoliths cover a long range of the oceanic stratigraphy deep down to the lithosphere/asthenosphere boundary, we present more detailed chemical and thermal state of the whole oceanic lithosphere in the presentation.