T256 Natural Carbon Dioxide Accumulations as Analogs for Geologic Storage

Tuesday, 21 October 2014
1:00 PM - 5:00 PM
205/206

LOW TEMPERATURE MAGNESITE FORMATION IN MG-CARBONATE PLAYAS: A NATURAL ANALOGUE FOR CO2 SEQUESTRATION

Paper number: T256-6
Program: Technical Sessions
Time: 2:05 PM – 2:20 PM

Authors

Abstract

Secure carbon storage via carbon mineralization requires an understanding of fundamental geochemical processes related to fluid-rock interactions at large spatial scales and over millennial timescales1. In northern British Columbia, hydromagnesite-magnesite playas (hectare-scale) have formed at the Earth’s surface since the last deglaciation (~11 ka), demonstrating the stability required for long-term carbon storage. Weathering of ultramafic bedrock produces Mg-rich groundwaters that discharge into topographic lows where the playas lie2. Over several millennia, geochemical, physical and microbial processes have mediated carbonate precipitation, with sediment deposition transitioning from siliciclastic to subaqueous Ca-Mg-carbonate precipitation to subaerial Mg-carbonate deposition3,4. A complex assemblage of carbonate minerals is present within the playas including magnesite [MgCO3], which is one of the most stable forms for CO2. Understanding the mechanisms for low temperature magnesite formation is relevant to ex situ and shallow in situ carbonation as a means of sequestering CO2. Magnesite precipitation at ambient temperatures is kinetically inhibited as a consequence of the strong hydration of Mg2+ ions in solution5. Consequently, understanding the rates of and controls on magnesite formation at low temperatures remains a challenge. Magnesite abundance is up to 40 wt.% at the surface and up to 86 wt.% at depth. Stable, radiogenic, and clumped isotope6 data as well as electron microscopy demonstrate that the magnesite is modern (<10,000 years) and formed at low temperatures (<15 °C) through direct precipitation in the shallow subsurface. At greater depths (~ 2 m), magnesite forms via diagenesis of Ca-carbonate sediments, demonstrating an alternate pathway for low temperature magnesite formation. The current focus is determining the rates of low-temperature magnesite formation, which has implications for the long-term storage of anthropogenic CO2 as Mg-carbonate minerals.

1Bickle et al. (2013) Rev. Mineral. Geochem. 77: 15-71. 2Power et al. (2009) Chem. Geol. 206: 302-316. 3Power et al. (2007) Geochem. Trans. 8: 13. 4Power et al. (in press) Sedimentology. 5Hänchen et al. (2008) Chem. Eng. Sci. 63: 1012-1028. 6Streit Falk and Kelemen, unpublished data.