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Cited by 250Geological and Geochemical AnalysisGeochemistry and Elemental AnalysisHigh-pressure geophysics and materials

Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions

Klein, F., Bach, W.·2009·Journal of Petrology
DOI 10.1093/petrology/egn071
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Abstract

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O. We have compiled and evaluated thermodynamic data for Fe–Ni–Co–O–S phases and computed phase relations in fO2,g–fS2,g and aH2,aq–aH2S,aq diagrams for temperatures between 150 and 400°C at 50 MPa. We use the relations and compositions of Fe–Ni–Co–O–S phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 15°20′N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awaruite–magnetite–pentlandite and heazlewoodite–magnetite–pentlandite assemblages forming in the early stages of serpentinization to millerite–pyrite–polydymite-dominated assemblages in steatized rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen, which leads to the formation of awaruite. Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial stage of serpentinization. In contrast, steatitization indicates increased silica activities and that high-sulfur-fugacity sulfides, such as polydymite and pyrite–vaesite solid solution, form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under these conditions, sulfides will not desulfurize but precipitate and the sulfur content of the rock increases. The co-evolution of fO2,g–fS2,g in the system follows an isopotential of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast, H2 in vent fluids is not buffered by Fe–Ni–Co–O–S phases, which merely monitor the evolution of H2 activities in the fluids in the course of progressive rock alteration. The co-occurrence of pentlandite–awaruite–magnetite indicates H2,aq activities in the interacting fluids near the stability limit of water. The presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds.

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Crossref · 10.1093/petrology/egn071

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  • The H2 in hydrothermal vents comes from serpentinization.

    “The H2 in hydrothermal vents comes from a geochemical process called serpentinization [21–25].”

    From Early bioenergetic evolution (2013) · cites this paper as [25]

  • Serpentinization occurs because suboceanic crust consists of iron-magnesium silicates such as olivine with Fe2+, and seawater at high pressure and around 150°C oxidizes Fe2+ to Fe3+ while water is reduced to H2.

    “Serpentinization takes place because most of the suboceanic crust consists of iron–magnesium silicates like olivine, in which the redox state of the iron is Fe2+. Seawater penetrates the ocean floor crust, reaching depths on the order of 3–5 km below the surface. There at high pressure and temperatures of around 150°C, water oxidizes the Fe2+ to Fe3+, the water being reduced to H2 with the oxygen”

    From Early bioenergetic evolution (2013) · cites this paper as [25]

  • In the temperature range 150-300°C, activities of H2 exceeding 200 mM are needed for awaruite to form.

    “The point is that in the temperature range 150–300°C, activities of H2 exceeding 200 mM are needed for awaruite to form [25].”

    From Early bioenergetic evolution (2013) · cites this paper as [25]

  • The presence of awaruite in serpentinizing systems indicates that at some point during their lifespan, serpentinizing systems regularly generate extremely reducing conditions on the order of greater than or equal to 200 mM H2, which would be conducive to organic synthesis.

    “The presence of awaruite in serpentinizing systems thus clearly indicates that at some point during their lifespan, serpentinizing systems regularly generate extremely reducing conditions on the order of greater than or equal to 200 mM H2, which would be conducive to organic synthesis, as Klein & Bach [25] point out.”

    From Early bioenergetic evolution (2013) · cites this paper as [25]

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Authors
Klein, F., & Bach, W.
Affiliations
University of Bremen (DE)
Corresponding author
Bach, W.
ORCID
Venue
Journal of Petrology
Publisher
Oxford University Press (OUP)
Published
2009-01-07
Volume
50
Issue
1
Pages
37-59
ISSN
  • 0022-3530 (ISSN-L)
  • 1460-2415
DOI
10.1093/petrology/egn071
OpenAlex
W2148570407
Cited by
250
References
3
Citations by year
2026: 12 · 2025: 9 · 2024: 13 · 2023: 16 · 2022: 17 · 2021: 23 · 2020: 18 · 2019: 21 · 2018: 10 · 2017: 20 · 2016: 10 · 2015: 15 · 2014: 14 · 2013: 12 · 2012: 11
FWCI
11.74
Type
journal article
Language
en
Open access
closed
Locations
Journal of Petrology (publishedVersion)
Topics
  • Physical Sciences › Earth and Planetary Sciences › Geophysics › Geological and Geochemical Analysis
  • Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Geochemistry and Elemental Analysis
  • Physical Sciences › Earth and Planetary Sciences › Geophysics › High-pressure geophysics and materials
Keywords
serpentinization, peridotite, oxygen fugacity, sulfur fugacity, hydrogen activity, awaruite, pentlandite, heazlewoodite, millerite, polydymite, pyrite, brucite, Mid-Atlantic Ridge, hydrothermal fluids, reducing conditions

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