[S-IT20] 地球深部科学

[SIT20-21] Cold SiO₂-rich slabs reaching the CMB revealed by the seifertite phase boundary

*新名 良介¹、河口 沙織²、佐藤 嶺³、大鶴 啓介³、河合 研志³、佐久間 博⁴、末原 茂 ⁴、石井 貴之⁵、米谷 珠萌¹ (1.明治大学、2.高輝度光科学研究センター、3.東京大学、4.物質・材料研究機構、5.岡山大学)

キーワード：SiO2、高温高圧力、X線回折測定、ザイフェルタイト

A cold silica (SiO₂)-rich subducted slab creates notable heterogeneity above the core-mantle boundary (CMB), influencing the physical and chemical evolution of the Earth’s mantle. This slab is expected to exhibit characteristic anomalies in the longitudinal and shear wave seismic velocity at the lowermost mantle, attributed to the SiO₂ phase transition into its dense polymorph, seifertite. However, the precise transition depth remains unclear because metastable phases are often observed in the SiO₂ system. To address this long-standing challenge, we conducted laser-heated diamond anvil cell experiments with synchronised rapid X-ray diffraction measurements alongside theoretical calculations. The results revealed that (1) the slope of the seifertite phase boundary was less steep than previously estimated and that (2) the temperature profile of the slab crosses the seifertite boundary twice, similar to the double-crossing in the post-perovskite transition. We observed a clear anti-correlation in seismic wave velocities matching the depth range of the seifertite transition in a cold slab beneath Central America. This is the first direct evidence that a cold SiO₂-rich slab descends towards the CMB. Additionally, the silica that exsolved from the ancient core may have initially been a more buoyant polymorph than seifertite.