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Title: Metallic iron limits silicate hydration in Earth’s transition zone

Abstract

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 andmore » the relatively low water content in bulk MTZ silicates inferred from electrical conductivity studies.« less

Authors:
ORCiD logo [1];  [1];  [1];  [1];  [2]
  1. Univ. of Michigan, Ann Arbor, MI (United States)
  2. Univ. of Illinois, Chicago, IL (United States)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
Contributing Org.:
Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
OSTI Identifier:
1575003
Grant/Contract Number:  
FG02-94ER14466; AC02-06CH11357; NA-0003858
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 116; Journal Issue: 45; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences
Country of Publication:
United States
Language:
ENGLISH
Subject:
58 GEOSCIENCES; mantle transition zone; water budget; redox reaction; mantle oxidation state; deep hydrogen cycle

Citation Formats

Zhu, Feng, Li, Jie, Liu, Jiachao, Dong, Junjie, and Liu, Zhenxian. Metallic iron limits silicate hydration in Earth’s transition zone. United States: N. p., 2019. Web. doi:10.1073/pnas.1908716116.
Zhu, Feng, Li, Jie, Liu, Jiachao, Dong, Junjie, & Liu, Zhenxian. Metallic iron limits silicate hydration in Earth’s transition zone. United States. https://doi.org/10.1073/pnas.1908716116
Zhu, Feng, Li, Jie, Liu, Jiachao, Dong, Junjie, and Liu, Zhenxian. 2019. "Metallic iron limits silicate hydration in Earth’s transition zone". United States. https://doi.org/10.1073/pnas.1908716116. https://www.osti.gov/servlets/purl/1575003.
@article{osti_1575003,
title = {Metallic iron limits silicate hydration in Earth’s transition zone},
author = {Zhu, Feng and Li, Jie and Liu, Jiachao and Dong, Junjie and Liu, Zhenxian},
abstractNote = {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.},
doi = {10.1073/pnas.1908716116},
url = {https://www.osti.gov/biblio/1575003}, journal = {Proceedings of the National Academy of Sciences of the United States of America},
issn = {0027-8424},
number = 45,
volume = 116,
place = {United States},
year = {Mon Oct 21 00:00:00 EDT 2019},
month = {Mon Oct 21 00:00:00 EDT 2019}
}

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Cited by: 14 works
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Works referenced in this record:

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