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| Titel |
Magnesite formation in playas: A natural analogue for carbon sequestration |
| VerfasserIn |
Ian Power, Anna Harrison, Siobhan Wilson, Gregory Dipple, Stewart Fallon |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104729
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| Publikation (Nr.) |
 EGU/EGU2015-4161.pdf |
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| Zusammenfassung |
| Non-marine carbonate deposits are of renewed interest as natural analogues for carbon
sequestration and storage. Specifically, the sequestration of anthropogenic carbon dioxide
(CO2)in Mg-carbonate minerals is being actively investigated as a strategy for reducing
greenhouse gas emissions1. In northern British Columbia, hydromagnesite-magnesite playas
(hectare-scale) have formed since the last deglaciation, suggesting that these minerals possess
a level of stability required for long-term carbon storage2. Quantitative mineralogical and
hydrogeochemical data, as well as microscopy and field observations, were used to formulate
a comprehensive facies model that describes the depositional environments for the formation
of these playas. Over several millennia, there have been transitions from deposition of
siliciclastic to subaqueous Ca-Mg-carbonate to subaerial Mg-carbonate sediments3,4.
Consequently, a complex assemblage of carbonate minerals is present within the playas
including magnesite [MgCO3], the most stable Mg-carbonate for storing CO2.
Magnesite precipitation at near-surface temperatures is kinetically inhibited due to
the strong hydration of Mg2+ ions in solution5. Thus, understanding the rates of,
and controls on, magnesite formation at low temperatures remains a challenge.
Magnesite abundances at the surface (1 to 41 wt.%) and at depth (1 to 86 wt.%) within
the playas are highly variable4. There is a propensity for hydrated Mg-carbonate
minerals to undergo transformation to less hydrated, more stable forms (lansfordite >
nesquehonite > dypingite > hydromagnesite)5; however, stable, radiogenic, and clumped
isotope6 data as well as electron microscopy demonstrate that magnesite formation
is likely dominated by direct precipitation from aqueous solution in the shallow
subsurface (~3-10 Ë C). An observed variation in magnesite crystal morphology with
depth is attributed to different crystal growth mechanisms induced by changes in
magnesite saturation state. Particle size analyses show a positive correlation between
magnesite abundance and mean particle size, indicating that magnesite formation is
primarily limited by nucleation rather than crystal growth kinetics. We estimate that
the rate of magnesite formation (nucleation + growth) is between 10-17 to 10-16
mol/cm2/s. Conversely, in the Ca-Mg-carbonate unit, magnesite may be forming via
diagenesis of Ca-carbonate minerals. Our continued focus is to further constrain
the rates and modes of magnesite formation in the context of long-term storage of
CO2.
[1] Power et al. (2013) Rev. Mineral. Geochem. 77: 305-360. [2] Power et al.
(2009) Chem. Geol. 206: 302-316. [3] Power et al. (2007) Geochem. Trans. 8: 13.
[4] Power et al. (2014) Sedimentology. 61:1701-1733. [5] Hänchen et al. (2008)
Chem. Eng. Sci. 63: 1012-1028. [6] Streit Falk and Kelemen, unpublished data. |
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