Subducted oceanic crust is enriched in free silica. Although being one of the silica polymorphs at lower-mantle pressures, niccolite-type phase (Nt-phase) has not been documented in multicomponent metabasaltic or metasediment compositions relevant to subducting oceanic crust. Here, we report the formation of an Al-rich Nt-phase (∼24.4 to 32.4 wt% Al₂O₃), coexisting with Al-depleted bridgmanite (∼6.4 to 7.6 wt% Al₂O₃), δ-phase, and iron-rich phase in model hydrated basalts over the pressure-temperature range of 84–113 GPa and 1,800–2,200 K. Infrared spectroscopy of a pure synthetic Al-rich Nt-phase shows OH bending and stretching vibrations at high pressures, indicative of its hydrous nature. This study suggests that Al-rich Nt-phase can serve as a potential water carrier in subducted oceanic crust to the deep lower mantle.

中文翻译：

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俯冲大洋地壳中富铝镍钴矿型二氧化硅的形成：对下地幔深部水输送的影响

俯冲洋壳富含游离二氧化硅。尽管是低地幔压力下的二氧化硅多晶型物之一，但在与俯冲洋壳相关的多组分变玄武质或变质沉积物组成中尚未记录到尼科莱特型相（Nt-相）。在这里，我们报告了富铝的 Nt 相（~24.4 到 32.4 wt% Al ₂ O ₃）的形成，与贫铝的 Bridgmanite（~6.4 到 7.6 wt% Al ₂ O _{3 ）共存})、δ 相和模型水合玄武岩中的富铁相在 84-113 GPa 和 1,800-2,200 K 的压力-温度范围内。纯合成富铝 Nt 相的红外光谱显示 OH 弯曲和拉伸振动在高压下，表明其含水性质。该研究表明，富铝 Nt 相可以作为俯冲洋壳到下地幔深处的潜在水载体。