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

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 deposition^3,4. 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 ⁶Streit Falk and Kelemen, unpublished data.