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| Titel |
Fe-Distribution and Hydrogen Generation During Serpentinization |
| VerfasserIn |
F. Klein, W. Bach, N. Jöns, T. McCollom, T. Berquó, B. Moskowitz |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026262
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| Zusammenfassung |
| Serpentinization of peridotite generates large amounts of dihydrogen (H2,aq), indicated by
the presence of Ni–Fe alloys and low-sulfur-fugacity sulfides, e. g. awaruite and pentlandite,
in serpentinites. Hydrogen is produced when ferrous iron in olivine is oxidized by water to
ferric iron in secondary magnetite and serpentine. This process is strongly dependent on bulk
rock composition, water-to-rock ratio and temperature. These relations were examined in
thermodynamic reaction path models (using the EQ3/6 computer code) with dunitic and
harzburgitic rock compositions.
The model results were compared with electron microprobe analyses, bulk magnetization
measurements, and Mößbauer spectroscopy of partially to fully serpentinized dunites and
harzburgites from Ocean Drilling Program Leg 209, Hole 1274A, Mid-Atlantic Ridge 15-N.
These samples have mesh rims that reveal a distinct in-to-out zoning, starting with brucite
(Mg# 80) at the interface with olivine, then a zone of serpentine (Mg# 95) + brucite ±
magnetite, and finally serpentine + magnetite in the outermost mesh rim. The composition of
co-existing serpentine and brucite in pseudomorphic mesh rims is virtually constant in most
samples from 32 to 147 meters below seafloor, suggesting similar alteration conditions of
olivine downhole.
Bulk magnetization measurements of microdrilled mesh rims in combination with
thin section petrography revealed a positive correlation of magnetite content with
extent of serpentinization. Where relic olivine is present, the magnetite content is
significantly lower then in fully serpentinized rocks. In these domains with sparse
magnetite, Mößbauer spectra revealed Fe3+/-
Fe values between 0.30 and 0.48
for paramagnetic minerals in the mesh rims (i. e., secondary hydrous phases). In
heavily to completely serpentinized rocks with abundant magnetite, Fe3+/-
Fe
values of the paramagnetic phases are consistently higher and range from 0.53 to
0.68.
In the EQ3/6 runs, a serpentine solid solution model that includes greenalite
and hisingerite (Fe2Si2O5(OH)4) was used in investigating the distribution of iron
between serpentine and magnetite and its oxidation state in serpentine. Our model
computations predict that above 330 -C and water activities near unity, the dissolution of
olivine and coeval formation of serpentine, magnetite and dihydrogen is significantly
obstructed by the dearth of silica. At these temperatures, hydrogen fugacities are
too low for awaruite and pentlandite to be stable. When temperatures drop below
320–330 -C, brucite becomes stable and hydrogen generation is facilitated, because the
reaction of olivine to serpentine, magnetite and brucite requires no external silica. The
MgO–FeO–Fe2O3–SiO2–H2O and Fe–Ni–Co–O–S phase relations observed in the
mesh rims suggest that serpentine and brucite from Hole 1274A likely formed at
temperatures between 150 and 250 -C and water-to-rock ratios (w/r) between 5
and 0.1. However, formation of awaruite must have taken place during main stage
serpentinization at temperatures between 200–250 -C and w/r < 1, when alteration
conditions were sufficiently reducing. Likewise, the model predicts the Fe3+/-
Fe ratios
of mesh-rim serpentine/brucite observed in incompletely serpentinized rocks of
serpentine (0.3 to 0.5) at low w/r ratios and T < 250 -C. The calculation results
furthermore indicate that elevated Fe3+/-
Fe ratios (0.5 to 0.7) measured in fully
serpentinized rocks appear to correspond to higher w/r ratios and less reducing
conditions.
Our study indicates that unprecedented details about the reaction sequences during
serpentinization may be obtained from merging careful petrographic, magnetic, and
spectroscopic analyses with comprehensive thermodynamic modeling. |
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