H2-rich fluids from serpentinization: geochemical and biotic implications

Our plain-English summary of H2-rich fluids from serpentinization: geochemical and biotic implications, by N.H. Sleep, A. Meibom, T. Fridriksson, et al (Proceedings of the National Academy of Sciences of the United States of America, 2004).

The study

• Authors: N.H. Sleep, A. Meibom, T. Fridriksson, R.G. Coleman, D.K. Bird
• Journal: Proceedings of the National Academy of Sciences of the United States of America
• Published: 2004;101(35):12818-23
• Cited by: 140 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Metamorphic hydration and oxidation of ultramafic rocks produces serpentinites, composed of serpentine group minerals and varying amounts of brucite, magnetite, and/or FeNi alloys. These minerals buffer metamorphic fluids to extremely reducing conditions that are capable of producing hydrogen gas. Awaruite, FeNi3, forms early in this process when the serpentinite minerals are Fe-rich. Olivine with the current mantle Fe/Mg ratio was oxidized during serpentinization after the Moon-forming impact. This process formed some of the ferric iron in the Earth's mantle. For the rest of Earth's history, serpentinites covered only a small fraction of the Earth's surface but were an important prebiotic and biotic environment. Extant methanogens react H2 with CO2 to form methane. This is a likely habitable environment on large silicate planets. The catalytic properties of FeNi3 allow complex organic compounds to form within serpentinite and, when mixed with atmospherically produced complex organic matter and waters that circulated through basalts, constitutes an attractive prebiotic substrate. Conversely, inorganic catalysis of methane by FeNi3 competes with nascent and extant life.

Copyright 2004 The National Academy of Sciencs of the USA

In plain English

This article is a geochemistry paper, not a study of asbestos and human health. The authors used equilibrium thermodynamic calculations to estimate dissolved hydrogen concentrations in water in equilibrium with serpentinite, the conditions for saturation with hydrogen gas, and where the nickel-iron mineral awaruite is stable. Serpentinite is a rock made largely of serpentine minerals, including chrysotile. No people, workers or health outcomes were studied. Chrysotile was treated only as a mineral: as a component in the modeled reactions and as a form of serpentine that grows in rock pore space. The calculations assumed a fluid pressure of 500 bars. Fluids in equilibrium with high-iron serpentinite were supersaturated with hydrogen gas above 160°C. Fluids in equilibrium with extensively oxidized serpentinite were undersaturated at all temperatures shown. Awaruite (FeNi3) could not form at 200°C unless the hydrogen partial pressure was more than 320 bars. The authors treated 50 bars as a reasonable upper limit for hydrogen after the Moon-forming impact. They concluded that significant oxidation of Earth's mantle could have occurred under hydrothermal conditions on the early Earth. They noted that their calculations left out sulfur and carbon species.

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: Theoretical study using equilibrium thermodynamic calculations (phase equilibria). It made no measurements on people, animals or field samples.
• Who was studied: No human or animal population was studied. The study modeled serpentinite mineral assemblages reacting with water, assuming 500 bars total fluid pressure. The assemblages were high-Fe and low-Fe serpentine with brucite and magnetite, and the same minerals with awaruite.
• Exposure: No human exposure was studied. The modeled process was the reaction of water with ultramafic rocks (olivine and pyroxene) to form serpentine (Mg-Fe chrysotile), brucite and magnetite, which produces hydrogen. Chrysotile is treated geochemically, as a component in the modeled reactions and as a serpentine form that grows in rock pore space. It is not treated as a source of exposure.
• Main results:
  • Fluids buffered by the high-Fe serpentinite assemblage (S0.1BM) were supersaturated with hydrogen gas at temperatures more than 160°C.
  • Fluids in equilibrium with extensively oxidized, low-Fe serpentinite (S0.025BM) were undersaturated with hydrogen gas at all temperatures shown.
  • At 200°C, awaruite of composition FeNi3 could not form unless the hydrogen partial pressure was more than 320 bars. The authors said this would preclude awaruite formation in a shallow land system.
  • The authors stated that hydrogen forms in molar to millimolar quantities at all temperatures, even after much of the iron in serpentine is oxidized.
  • From mass balance, the authors estimated an equivalent pressure of 50 bars as a reasonable upper limit for hydrogen in the aftermath of the Moon-forming impact.
• The authors' conclusion: The authors concluded that significant oxidation of the mantle could have occurred under hydrothermal conditions on the early Earth, or on other bodies where ultramafic rocks covered the surface.
• Limitations the authors state:
  • The calculations do not include carbon species, so they do not apply to a massive CO2 atmosphere.
  • Sulfur was ignored in the calculations.
  • Hydrogen was assumed to behave as an ideal gas.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Serpentinization, Ultramafic Rocks, Serpentinite, Serpentine Group, Brucite, Olivine, Silicate.

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