Japan Geoscience Union Meeting 2019

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-GC Geochemistry

[S-GC40] Volatile Cycles in the Deep Earth - from Subduction Zone to Hot Spot

Wed. May 29, 2019 1:45 PM - 3:15 PM A10 (TOKYO BAY MAKUHARI HALL)

convener:Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo), Gray E Bebout(Lehigh University), Chairperson:Takeshi Hanyu, Gray Bebout(Lehigh University), Yuji Sano, Hirochika Sumino

2:45 PM - 3:00 PM

[SGC40-04] Formation of Fe-rich volatile-bearing phases in the deep lower mantle and the implications for deep Earth volatile cycles

*Li Zhang1, Hongsheng Yuan1, Eiji Ohtani2, Lianjie Man1, Duck Young Kim1, Dmitry Popov3, Yue Meng3, Eran Greenberg4, Vitali Prakapenka4 (1.Center for High Pressure Science and Technology Advanced Research (HPSTAR), 2.Department of Earth Science, Graduate School of Science, Tohoku University, 3.HPCAT, X-ray Science Division, Argonne National Laboratory, 4.GeoSoilEnviroCARS, University of Chicago)

Keywords:deep lower mantle, volatile cycle, post-perovskite, synchrotron X-ray diffraction

Inclusions in deep diamonds provide a window to the actual deep mantle processes. The observation of hydrous ringwoodite with ~1 wt.% H2O as a diamond inclusion indicates the transition zone is, at least locally, very wet [1]. Furthermore, the recent discovery of Ice-VII and halite inclusions at pressures as high as 24 GPa in diamonds provides direct evidence for the existence of saline fluid at least down to the shallow lower mantle [2]. To understand the interaction of water and Cl with the Fe-bearing lower mantle, we performed experiments to simulate the behavior of volatile-bearing systems in laser-heated diamond anvil cells (DACs) under high pressure-temperature (P-T) conditions corresponding to the deep lower mantle. The phase assemblages were determined by a combination of in-situ synchrotron-based X-ray diffraction (XRD) and ex situ transmission electron microscope (TEM) analysis. In the system MgO-Al2O3-Fe2O3-SiO2-H2O containing ~7 wt.% water, the hydrous Fe-bearing δ-phase coexists with both bridgmanite (Bdg) and post-perovskite (pPv) in a broad P-T range of 104-126 GPa and 1900-2500 K, whereas the pyrite-type (py) FeOOH phase was observed coexisting with the pPv phase. In situ XRD data further revealed that saline fluid reacts with Fe-bearing pPv to form a previously unknown cubic phase of FeCl2 that adopts the identical space group of Pa3 with the py-phase FeOOH. The TEM chemical analysis revealed that the composition of the pPv phase contains 3.5 wt.% Na2O and 16.5 wt.% FeO after the reaction with the oversaturated saline fluid. Formation of the very dense Fe-rich volatile-bearing phases in the lowermost mantle provides clues for the deep storage of water and Cl and may contribute to the chemical heterogeneities in this region.
[1] D. G. Pearson et al., Nature 507 (2014).
[2] O. Tschauner et al., Science 359 (2018).