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 . 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 . 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.
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 O. Tschauner et al., Science 359 (2018).