[S-CG63] 沈み込み帯へのインプット：海洋プレートの進化と不均質

[SCG63-08] カーボナタイトフラックスに起因するアセノスフェア部分溶融およびプチスポットマグマのリソスフェアへのインプット

*三國 和音¹、平野 直人¹、秋澤 紀克²、町田 嗣樹³、角野 浩史²、田村 明弘⁴、森下 知晃⁴ (1.東北大学、2.東京大学、3.千葉工業大学、4.金沢大学)

キーワード：プチスポット火山、カーボナタイト、アセノスフェア、リソスフェア、太平洋プレート

The lithosphere–asthenosphere boundary (LAB), seismically detected, stabilizes plate tectonics. The cause of LAB discontinuity is an open question because conflicting hypotheses are proposed, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts, ascent from the asthenosphere owing to subducting plate flexure, plausibly supports the partial melting at LAB. However, the partial melting in the asthenosphere requires appropriate exotic materials (e.g., volatiles) to lower the solidus temperature. In this study, we observed lava outcrops of the six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. The ultramafic xenoliths were also obtained by a petit-spot knoll in this region (Mikuni et al., 2022). ⁴⁰Ar/³⁹Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic compositions were obtained for the petit-spot basalts in this study. The ⁴⁰Ar/³⁹Ar ages for the two monogenetic volcanoes composed of crystalline basalts are ca. 2.6 Ma and ca. 0 Ma indicating that the western Pacific petit-spot volcanoes erupted owing to the flexure of subducting Pacific Plate into Mariana Trench within 3 Ma. The isotopic compositions of the western Pacific petit-spot basalts indicate that their geochemically identical melting source, and that is explained by a mixture of the HIMU-like and the EM-1-like components. The mass balance-based melting models for multi trace elements imply that the characteristic trace element composition (i.e., Zr, Hf, Ti depletions) of petit-spot magmas were derived from the partial melting of garnet lherzolite with a low carbonatite influx and a small amount of crustal components. This result quantitatively defines the involvement of carbonatite melt and recycled crust for the petit-spot melts with other examples of tectonic-induced volcanisms (e.g., Hawaiian North Arch lava) showing similar geochemical signatures to those of petit-spots. Moreover, the carbonatite melt and recycled oceanic crust could be closely related to the HIMU and EM-1 components. The mineral chemistry and geothermobarometry of the depleted/enriched peridotite and fertile pyroxenites suggest that the lithospheric mantle of the oldest portion in Pacific Plate would be locally modified during the melt ascends through the lithosphere. The expected occurrence of pyroxene-rich metasomatic vein provides the implication for the physical property of subducting lithosphere into Mariana Trench.

Reference
Mikuni et al. (2022) Prog. Earth Plant. Sci., 9, 62. https://doi.org/10.1186/s40645-022-00518-y