Mantle hydration along outer-rise faults inferred from serpentinite permeability

Our plain-English summary of Mantle hydration along outer-rise faults inferred from serpentinite permeability, by K. Hatakeyama, I. Katayama, K.I. Hirauchi, et al (Scientific Reports, 2017).

The study

• Authors: K. Hatakeyama, I. Katayama, K.I. Hirauchi, K. Michibayashi
• Journal: Scientific Reports
• Published: 2017;7(1):13870
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Recent geophysical surveys indicate that hydration (serpentinization) of oceanic mantle is related to outer-rise faulting prior to subduction. The serpentinization of oceanic mantle influences the generation of intermediate-depth earthquakes and subduction water flux, thereby promoting arc volcanism. Since the chemical reactions that produce serpentinite are geologically rapid at low temperatures, the flux of water delivery to the reaction front appears to control the lateral extent of serpentinization. In this study, we measured the permeability of low-temperature serpentinites composed of lizardite and chrysotile, and calculated the lateral extent of serpentinization along an outer-rise fault based on Darcy's law. The experimental results indicate that serpentinization extends to a region several hundred meters wide in the direction normal to the outer-rise fault in the uppermost oceanic mantle. We calculated the global water flux carried by serpentinized oceanic mantle ranging from 1.7 × 1011 to 2.4 × 1012 kg/year, which is comparable or even higher than the water flux of hydrated oceanic crust.

In plain English

This was a geology study, not a medical one. It did not study human health or disease; chrysotile appears only as a mineral in the rocks tested. The authors measured the permeability (how easily fluid passes through) of seven samples of low-temperature serpentinite made mainly of lizardite and chrysotile. The samples came from the Mineoka Belt in Japan and the deep seafloor of the Parece Vela Basin, South Mariana Trench and Tonga Trench. Permeability, tested at 5 to 100 MPa of pressure, fell as pressure rose. The authors then used these measurements to calculate how far seawater could turn ocean mantle rock into serpentinite beside faults that form before the plate sinks into the Earth. Assuming 1.0 million years of water access, the calculated width 7 km below the seafloor ranged from 120 to 1390 m; shorter access gave narrower zones. The calculated global water flux ranged from 1.7 × 10 to the 11th to 2.4 × 10 to the 12th kg per year. The authors concluded this is comparable to or higher than the flux carried by hydrated ocean crust.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory measurement of rock permeability, followed by calculations based on Darcy's law (a geoscience study; no human or animal subjects)
• Who was studied: Seven serpentinite rock samples (per Table 1): four from Kamogawa and Sengen in the Mineoka Belt, Japan, and three dredged from the deep seafloor at the Parece Vela Basin, South Mariana Trench and Tonga Trench. No people were studied.
• Exposure: No human exposure was studied. The material tested was low-temperature serpentinite composed of lizardite and chrysotile, with minor magnetite and calcite. Permeability was measured with nitrogen gas at confining pressures of 5 to 100 MPa at room temperature.
• Main results:
  • At 100 MPa confining pressure, the Mineoka Belt (accretionary prism) samples showed variable permeability, from 10 to the minus 21st to 10 to the minus 19th square meters. The dredged seafloor samples had similar permeability, about 10 to the minus 20th square meters.
  • Extrapolated to uppermost-mantle conditions 7 km below the seafloor, serpentinite permeability was estimated at about 10 to the minus 23rd to 10 to the minus 21st square meters.
  • Using the intermediate (Tonga Trench) permeability and 1.0 million years of water supply, serpentinization was calculated to extend to a zone 790 m wide at 7 km below the seafloor. The zone narrowed to 200 m at about 12 km depth.
  • Across all seven samples, the calculated width of serpentinization at 7 km depth ranged from 120 to 1390 m. Table 2 gives these values for an assumed 1.0 million years of water access.
  • The calculated width depends on how long water has access. For 1.0 million years it was about three times larger than for 0.2 million years.
  • Across the samples, the calculated average water content of the top 5 km of the uppermost mantle ranged from 0.3 to 4.7 wt%.
  • The calculated global water flux carried by serpentinized oceanic mantle ranged from 1.7 × 10 to the 11th to 2.4 × 10 to the 12th kg per year.
• The authors' conclusion: The authors concluded that the global water flux carried by serpentinized oceanic mantle is comparable to or higher than the water flux of hydrated oceanic crust.
• Limitations the authors state:
  • The experiments reached only 100 MPa, while the uppermost mantle is above 200 MPa, so permeability had to be extrapolated.
  • The water flux estimate has a large uncertainty, including the development of outer-rise faults and local variations in the oceanic lithosphere.
  • The model assumed water delivery controls hydration, but hydration is also affected by reaction kinetics.
  • Measurement uncertainty is larger when fluid flux is extremely low, that is, for permeability below 10 to the minus 20th square meters.
  • Porosity for the Kamogawa sample was set to a constant value because its porosity measurements were highly uncertain.

Topics

Medical terms in this study, explained in our Lexicon: Serpentinite, Serpentinization, Lizardite, Chrysotile.

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