北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

9.2 OHTANI Eiji

Role of volatiles transported into the mantle transition zone and lower mantle

Eiji Ohtani

Graduate School of Science, Tohoku Univ.


Geophysical observations such as seismic wave velocity and electrical conductivity profiles suggest existence of the stagnant slabs in the wet mantle transition zone (1). It was also suggested by mineralogical study on hydrous mineral inclusions in diamond such as hydrous ringwoodite (2), hydrous minerals such as hydrous phase EGG and hydrous phase d (3) Aluminous mineral inclusions such as phase EGG, phase d, and phase TAPP (4) reported as inclusions in diamond might be continental and/or oceanic crustal components subducted in the wet mantle transition zone contains. Fluids or volatile-rich magmas may exist at the top of the lower mantle due to crossing of the convective descent of the cold hydrated slabs showing a large contrast of water contents between the mineral assemblages in the mantle transition zone and those of the lower mantle. Based on the recent study of the phase relations on hydrous peridotite, dense magmas are not likely to be formed at this depth and the hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported further into the lower mantle by collapse of the stagnant slabs, which supply water into the lower mantle and the core-mantle boundary. Hydrous phase d-H solid solution may be the most important hydrous phase in lower mantle (5). Existence of this phase reduces the aluminum content in coexisting bridgmantite and post-perovskite, and thus modifies the physical properties of the lower mantle. The iron-water reaction at the core-mantle boundary can create pyrite-type FeOOH which can be a potential candidate material for ULVZ (6, 7). Thus, water plays important roles on structure and dynamics of the mantle transition zone and the lower mantle.


References:

(1) Utada et al. (2009). Earth Planet. Sci. Lett. 281, 249–257.

(2) Pearson et al. (2014). Nature, 507, 221. doi:10.1038/nature13080.

(3) Wirth et al. (2007). Earth Planet. Sci. Lett., 259, 384–399.

(4) Armstrong and Walter (2012). Eur. J. Mineral. 2012, 24, 587–597.

(5) Ohira et al. (2014). Earth Planet. Sci. Lett., 401, 12–17. (6) Liu et al. (2017) Nature, 551(7681), 494. (7) Liang et al. (2018) Geophys. Res. Lett., in press.