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      Mineralogy of the deep lower mantle in the presence of H2O

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          Abstract

          Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2Hx (x≤1) phase. The (Mg, Fe)O2Hx has the pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2Hx is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+, and H+). This important phase has a number of far-reaching implications including the extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM, and an internal source for modulating oxygen in the atmosphere.

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          The rise of oxygen in Earth's early ocean and atmosphere.

          W Lyons, N Planavsky, T. Reinhard (2014)
          The rapid increase of carbon dioxide concentration in Earth's modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.
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            Post-perovskite phase transition in MgSiO3.

            Motohiko Murakami, Kei Hirose, Katsuyuki Kawamura … (2004)
            In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D" seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D" discontinuity.
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              Hydrous mantle transition zone indicated by ringwoodite included within diamond.

              D Pearson, F Brenker, F Nestola … (2014)
              The ultimate origin of water in the Earth's hydrosphere is in the deep Earth--the mantle. Theory and experiments have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth's transition zone, at depths between 410 and 660 kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine--wadsleyite and ringwoodite--to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results, and the issue of whether the transition zone contains abundant water remains highly controversial. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.
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                Author and article information

                Journal
                Title: National Science Review
                Publisher: Oxford University Press (OUP)
                ISSN (Print): 2095-5138
                ISSN (Electronic): 2053-714X
                Publication date Created: May 13 2020
                Publication date (Electronic): May 13 2020
                Affiliations
                [1 ]Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094, PRC
                [2 ]Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA
                [3 ]Center for Study of Matter under Extreme Conditions, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33199, USA
                [4 ]High Pressure Collaborative Access Team (HPCAT), X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
                [5 ]Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60437, USA
                Article
                DOI: 10.1093/nsr/nwaa098
                SO-VID: 3c0c2df7-8864-4601-b47c-423f03d90e73
                Copyright © © 2020
                License:

                http://creativecommons.org/licenses/by/4.0/

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