講演情報
[R7P-01]Mineralogical and Geochemical characteristics of hidden magmatism in old oceanic lithosphere revealed by International Ocean Drilling Programme (IODP3) Expedition 502
*秋澤 紀克1、三國 和音2、Lington Jennifer3、Yen Christopher4、宮崎 隆5、Narduzzi Francesco6、鹿児島 渉悟7、Preece Katie8、植田 勇人9、Vanucchi Paola10、Atton Lottie10、山口 飛鳥10、北島 弘子10、平野 直人10、IODP3 Exp502 Scientist10 (1. 広島大学、2. 産業技術総合研究所、3. University of Southampton、4. University of Lausanne、5. 海洋研究開発機構、6. University of Pavia、7. 富山大学、8. Swansea University、9. 新潟大学、10. その他)
キーワード：
プチスポット、太平洋プレート、沈み込み、マントル不均質
Petit-spot volcanism occurs on the flexed Pacific Plate before subduction and represents low-degree partial melting beneath old oceanic lithosphere. Although petit-spot volcanoes have been studied by dredged samples and submersible observations, direct drilling of in-situ volcanic samples had not been achieved until IODP3 Expedition 502. During this expedition, basaltic fragments were successfully recovered from Site C0027 in the northwestern Pacific, providing the first drilled samples of petit-spot volcanism. In this study, we investigated the petrography, mineral chemistry, and whole-rock geochemistry of the recovered basaltic samples to constrain magma evolution and its source characteristics. The recovered samples consist mainly of fresh glassy basalt and cryptocrystalline basalt with olivine and clinopyroxene phenocrysts. The groundmass commonly exhibits acicular to dendritic textures with glass, indicating rapid quenching after eruption. Vesicles are generally sparse and mostly unfilled, suggesting limited volatile exsolution and minimal post-eruptive alteration.Olivine phenocrysts show high Mg# [Mg/(Mg+Fe), up to ~0.86] and elevated NiO contents (~0.45 wt%), indicating crystallization from relatively primitive magma. Clinopyroxene compositions show variable Mg# ranging from 0.48 to 0.82, reflecting progressive magmatic differentiation. Bulk-rock trace-element patterns are characterized by negative Th–U–Ta–Nb anomalies and relatively weak negative Zr–Hf anomalies compared with previously reported petit-spot basalts. These geochemical features provide new mineralogical constraints on magma generation beneath old oceanic lithosphere and offer important insights into the origin and evolution of petit-spot magmatism associated with plate flexure prior to subduction.