 |
| Titel |
Mineral replacements during carbonation of peridotite: implications for carbon dioxide sequestration in ultramafic rocks |
| VerfasserIn |
Andreas Beinlich, Jörn Hövelmann, Oliver Plümper, Håkon Austrheim |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042869
|
|
|
|
|
|
| Zusammenfassung |
| In contact with CO2, ultramafic rocks are known to be reactive and eventually form
ophicarbonates and listwaenites. Here we present observations from serpentinized peridotite
clasts from the Solund Devonian Basin, SW Norway. These clasts show evidence for
a stepwise reaction history starting with initial serpentinization and resulting in
the formation of carbonates (mainly calcite and dolomite) and quartz. Thus, they
represent a natural analogue for CO2 sequestration in ultramafic rocks, which was
proposed by the Inter Governmental Panel on Climate Change (IPCC 2005) as one
possibility for long-term CO2 storage. In several layers of the basin, the carbonatized
ultramafic clasts are important constituents and account for up to 20 vol. % of the basin
infill.
The investigated clasts show a concentric build-up with green to grey colored cores
surrounded by mm to 10 cm thick zones of red to black shades. Textural evidence indicates
the following alteration sequence: An early stage is represented by serpentinization of
peridotite resulting in a typical mesh texture, with veins of serpentine and Ni-rich hematite
surrounding compartments of relict olivine (Fo90). Subsequently, relict olivine breaks down
to form an alteration product which is significantly depleted in Mg relative to the precursor
olivine. In the more advanced ophicarbonate stage, compartments are filled with calcite,
quartz, and talc. In the most advanced stage, quartz, calcite, and hematite dominate and occur
together with minor amounts of chromite, talc, and chlorite. The textural evolution is
accompanied by a decrease in whole-rock MgO from 40 to 2 wt. % and a CaO increase
from 1 to 35 wt. %. All clasts are characterized by high Cr and Ni (1000-4000
and 500-3000 ppm, respectively) revealing their ultramafic origin. Transmission
electron microscopy (TEM) observations indicate that the alteration product after
olivine is composed of an amorphous material, which is compositionally close to
serpentine, together with poorly crystalline serpentine and extremely fine grained
talc. Hydrothermal batch experiments (130-160 bar PCO2; 200Ë C; 1-3 weeks
reaction time) show that the alteration product after olivine is the favorable site of
reaction presumably due to the large reactive surface area. In contrast, the olivine
relicts have reacted to a significantly lesser extend, whereas the serpentine veins
remain virtually unreacted. The dissolution of the compartment fillings is followed by
nucleation and growth of calcite crystals also revealing that precipitation of calcite is
strongly favored over magnesite as soon as the system contains Ca. The preferred
precipitation of calcite is also supported by geochemical modeling (using Phreeqc), which
shows that the Mg-bearing carbonates (dolomite, magnesite) only form if the fluid is
sufficiently depleted in Ca. The compositional and textural differences between
different samples as well as different run products from experiments indicate that
the described clasts evolved from peridotite due to extreme mobilization of Mg,
development of secondary porosity, and infill of carbonates. Mg removed from the clasts
is partly consumed by replacement reactions in the vicinity of the clasts where
Fe-minerals (almandine) are altered to Mg-minerals (talc). For basins containing
abundant peridotite clasts, the outlined process will influence the CO2 and MgO
budget.
References:
IPCC Special report: Carbon Dioxide Capture and Storage, Summary for Policymakers,
2005. |
|
|
|
|
|
|