[S-CG67_1PM1] 海洋底地球科学

[SCG67-02] ルイビル海山列が深部マントル起源である証拠

*羽生 毅¹ (1.海洋研究開発機構)

キーワード：ルイビル海山, マントルプルーム, 深部マントル, 希ガス, 海洋掘削

Louisville seamount chain has been considered to be one of the long-lived hotspot tracks on the Pacific plate, but its magmatic source has not been well understood. I report noble gas compositions of the drill cores from four of the five seamounts drilled during IODP Expedition 330. Because the samples are aged (50-74 Ma), a stepwise crushing test for noble gas extraction from olivine phenocrysts and submarine glasses was made to assess contamination of post-eruption radiogenic nuclides. This test confirmed extraction of magmatic noble gases with minimal release of post-eruption radiogenic nuclides from the olivine samples; however, this was not always the case for the glass samples. The 3He/4He ratios in the olivine phenocrysts range from a value similar to those of mid-ocean ridge basalts (MORB) to slightly elevated ratios up to 10.6 Ra. Although these ratios are not as high as those observed in other ocean island basalts, two Louisville seamount basalts exhibit a primordial Ne isotopic signature that can be clearly discriminated from MORB Ne. The He and Ne isotopic compositions of the Louisville seamount basalts can be explained by the mixing of less degassed mantle and depleted upper mantle with different He/Ne ratios. The presence of the less degassed mantle component in the source of the Louisville seamounts documents the deep origin of the mantle plume.One of the major objectives of the IODP Expedition 330 was to test the geodynamic model that predicts lateral advection of mantle plumes in the convecting mantle [Koppers et al., G-cubed, 2004; Steinberger and Antretter, G-cubed, 2006]. This model assumes a primary mantle plume that is rooted deep in the mantle, and the trajectory of a plume conduit is influenced by the overall mantle flow, which can be monitored by the hotspot drift on the Earth's surface [Tarduno et al., Science, 2009]. The paleomagnetic and dating results from IODP Expedition 330 verified the geodynamic modeling predictions for the Louisville seamount chain together with the Hawaiian-Emperor seamount chain [Koppers et al., Nat. Geosci., 2012]. The present noble gas data gives a guarantee for the deep-rooted Louisville plume assumed in the geodynamic model.