Insights on the deep carbon cycle from the electrical conductivity of carbon-bearing aqueous fluids

Our plain-English summary of Insights on the deep carbon cycle from the electrical conductivity of carbon-bearing aqueous fluids, by G. Manthilake, M. Mookherjee, N. Miyajima (Scientific Reports, 2021).

The study

• Authors: G. Manthilake, M. Mookherjee, N. Miyajima
• Journal: Scientific Reports
• Published: 2021;11(1):3745
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The dehydration and decarbonation in the subducting slab are intricately related and the knowledge of the physical properties of the resulting C-H-O fluid is crucial to interpret the petrological, geochemical, and geophysical processes associated with subduction zones. In this study, we investigate the C-H-O fluid released during the progressive devolatilization of carbonate-bearing serpentine-polymorph chrysotile, with in situ electrical conductivity measurements at high pressures and temperatures. The C-H-O fluid produced by carbonated chrysotile exhibits high electrical conductivity compared to carbon-free aqueous fluids and can be an excellent indicator of the migration of carbon in subduction zones. The crystallization of diamond and graphite indicates that the oxidized C-H-O fluids are responsible for the recycling of carbon in the wedge mantle. The carbonate and chrysotile bearing assemblages stabilize dolomite during the devolatilization process. This unique dolomite forming mechanism in chrysotile in subduction slabs may facilitate the transport of carbon into the deep mantle.

In plain English

This was a laboratory geology experiment, not a health study. It does not address asbestos exposure or disease in people. The authors heated two natural serpentinite rock samples under high pressure (4 GPa) and measured how well they conducted electricity as they released fluids. One sample contained chrysotile with minor calcite (a carbonate). The other, used as a carbon-free comparison, contained antigorite with minor magnetite. Conductivity rose sharply at about 925 K for the chrysotile sample and at 850 K for the antigorite sample, as the minerals broke down to olivine and talc and released fluid. The fluid from the carbonate-bearing chrysotile showed more than one order of magnitude greater conductivity than the carbon-free fluid. In the chrysotile sample the authors found extensive dolomite crystals and carbonaceous material. Raman analysis identified graphite and diamond, plus C-H bands that the authors said likely indicate methane. The authors concluded that this carbon-bearing fluid can be an excellent indicator of carbon moving in subduction zones, and that dolomite formation in chrysotile may help carry carbon into the deep mantle.

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 high-pressure, high-temperature experiment measuring the electrical conductivity of rock samples (geoscience; no human or animal subjects)
• Who was studied: Two natural serpentinite rock samples: one containing chrysotile with minor calcite (supplied by the Mineralogical Museum of the University of Tubingen, Germany), and one consisting dominantly of antigorite with minor magnetite (the carbon-free control)
• Exposure: Samples held at 4 GPa pressure and heated step by step past the point where the hydrous and carbonate minerals break down and release fluid. Electrical conductivity was measured during heating and cooling.
• Main results:
  • A sharp rise in conductivity on dehydration occurred at 850 K in the antigorite sample and at about 925 K in the chrysotile-plus-calcite sample
  • Fluids released from both serpentine samples conducted about 2 orders of magnitude better than the host minerals
  • The carbon-bearing (C-H-O) fluid from the chrysotile sample showed more than one order of magnitude greater conductivity than the carbon-free fluid from the antigorite sample
  • Both serpentine samples broke down to olivine and talc. The chrysotile sample also showed extensive dolomite crystallization, which the antigorite sample did not.
  • Raman analysis of the chrysotile sample after dehydration confirmed diamond and showed graphite
  • Raman spectra of the chrysotile sample also showed C-H stretching bands, which the authors said likely indicate methane
  • Wetting angles at olivine-fluid-olivine boundaries were 45 to 60 degrees in the chrysotile sample and 0 to 10 degrees in the antigorite sample
• The authors' conclusion: The authors concluded that the carbon-bearing fluid from carbonated chrysotile conducts electricity better than carbon-free fluid and can be an excellent indicator of carbon migration in subduction zones. They also concluded that dolomite formation in chrysotile may facilitate carbon transport into the deep mantle.
• Limitations the authors state:
  • The authors note that chrysotile is metastable (less stable than lizardite) but chose it because their focus was the fluid's conductivity
  • The authors suggest low-temperature conduction may reflect protons in grain boundaries that were not fully removed when the sample was dried

Topics

Medical terms in this study, explained in our Lexicon: Decarbonation, Serpentine Group, Chrysotile, Dolomite.

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