Metallic iron limits silicate hydration in Earth’s transition zone

The Earth’s mantle transition zone (MTZ) is often considered an internal reservoir for water because its major minerals wadsleyite and ringwoodite can store several oceans of structural water. Whether it is a hydrous layer or an empty reservoir is still under debate. Previous studies suggested the M...

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Published inProceedings of the National Academy of Sciences - PNAS Vol. 116; no. 45; pp. 22526 - 22530
Main Authors Zhu, Feng, Li, Jie, Liu, Jiachao, Dong, Junjie, Liu, Zhenxian
Format Journal Article
LanguageEnglish
Published United States National Academy of Sciences 05.11.2019
Subjects
deep hydrogen cycle
Diamonds
Earth mantle
Electrical conductivity
Electrical resistivity
Electrical studies
Enrichment
GEOSCIENCES
Hydration
Hydrogen reduction
Hydrogen storage
Iron
mantle oxidation state
mantle transition zone
Metals
Minerals
Moisture content
Oceans
Physical Sciences
redox reaction
Reservoirs
Silica
Silicates
Slabs
Transition zone
water budget
Water content
Water distribution
Water engineering
mantle transition zone
mantle oxidation state
water budget
deep hydrogen cycle
redox reaction
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Summary:The Earth’s mantle transition zone (MTZ) is often considered an internal reservoir for water because its major minerals wadsleyite and ringwoodite can store several oceans of structural water. Whether it is a hydrous layer or an empty reservoir is still under debate. Previous studies suggested the MTZ may be saturated with iron metal. Here we show that metallic iron reacts with hydrous wadsleyite under the pressure and temperature conditions of the MTZ to form iron hydride or molecular hydrogen and silicate with less than tens of parts per million (ppm) water, implying that water enrichment is incompatible with iron saturation in the MTZ. With the current estimate of water flux to the MTZ, the iron metal preserved from early Earth could transform a significant fraction of subducted water into reduced hydrogen species, thus limiting the hydration of silicates in the bulk MTZ. Meanwhile, the MTZ would become gradually oxidized and metal depleted. As a result, water-rich region can still exist near modern active slabs where iron metal was consumed by reaction with subducted water. Heterogeneous water distribution resolves the apparent contradiction between the extreme water enrichment indicated by the occurrence of hydrous ringwoodite and ice VII in superdeep diamonds and the relatively low water content in bulk MTZ silicates inferred from electrical conductivity studies.
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FG02-94ER14466; AC02-06CH11357; NA-0003858
USDOE Office of Science (SC)
3Present address: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Edited by Ho-Kwang Mao, Center for High Pressure Science and Technology Advanced Research, Shanghai, China, and approved September 27, 2019 (received for review May 21, 2019)
2Present address: Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Mānoa, Honolulu, HI 96822.
Author contributions: F.Z. and J. Li designed research; F.Z., J. Li, J. Liu, J.D., and Z.L. performed research; F.Z., J. Li, and Z.L. analyzed data; and F.Z. and J. Li wrote the paper.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1908716116