ε-FeOOH在中下地幔储存的证据及其对水循环的意义

Mid-mantle water transportation implied by the electrical and seismic properties ofε-FeOOH

地球内部转换带与核幔边界的水对地球的分层、元素循环以及地核组成等方面都有十分重要的影响.如果水是在地球表面与内部圈层间循环的话,那么理论上在下地幔也应该有水的通道,而且该水通道的地震地磁学特征应有别于其周围的下地幔环境.在本次工作中,利用动静高压结合的方法(二级轻气炮+金刚石压腔)并综合理论计算系统研究了ε-FeOOH在下地幔温压范围内的波速与电导率剖面及其内部机制.我们发现,在50 GPa左右由于铁的电子自旋相变的影响,ε-FeOOH的电导率有3个数量级的急剧升高而其纵波波速会突然下降16.8%左右.结合全球地震地磁学数据分析,ε-FeOOH在这个压力范围的高导低波现象恰好与全球范围内在1200 km深度左右的某些高导低波的冷俯冲带区域相吻合.含水ε-FeOOH或者其他含铁相可能具有的这些独特性质或许可以作为水在中下地幔通过的证据.

Water in the mantle transition zone and the core-mantle boundary plays a key role in Earth’s stratification,volatile cycling,and core formation.If water transportation is actively running between the aforementioned layers,the lower mantle should contain water channels with distinctive seismic and/or electromagnetic signatures.Here,we investigated the electrical conductivity and sound velocity ofε-FeOOH up to 71 GPa and 1800 K and compared them with global tomography data.An abrupt threeorder jump of electrical conductivity was observed above 50 GPa,reaching 1.24(12)×10^(3)S/m at 61 GPa.Meanwhile,the longitudinal sound velocity dropped by 16.8%in response to the high-to-low spin transition of Fe^(3+).The high-conductivity and low-sound velocity ofε-FeOOH match the features of heterogenous scatterers in the mid-lower mantle.Such unique properties of hydrousε-FeOOH,or possibly other Fe-enriched phases can be detected as evidence of active water transportation in the mid-lower mantle.