CARBONATE ALTERED SERPENTINITES OF ATLIN, BC: A TWO STEPPED ANALOG TO CO2 SEQUESTRATION

The Kyoto Protocol, calls for the industrialized world to reduce greenhouse gas emissions, especially CO₂, 5% below 1990 levels from 2008-2012.  CO₂ sequestration by carbonation of magnesium silicate minerals is one storage mechanism being considered for implementation on an industrial scale.  Mineral carbonation binds CO₂ into the lattice of carbonate minerals forming stable and environmentally benign by-products.  Although the reaction has proven difficult to induce in the laboratory, the rock record of the process is ubiquitous in serpentinite terrains.  Our study of these natural mineral carbonation systems will identify the accelerated pathways and catalysts as well as physical and chemical environments that promote reaction.

Fossil mineral carbonation systems, which occur as magnesite-talc-quartz alteration of serpentinite, are well-exposed near Atlin B.C. They are low temperature (~200°C), low salinity systems which occur peripheral to fault and fracture systems within the Atlin ophiolitic assemblage. Whole-rock geochemical analyses indicate that complete carbonation of serpentinite was accompanied by a mass gain of 35 ± 6%.  This and the observed mineralogy confirm that carbonation at Atlin was analogous to the reaction proposed for mineral carbonation sequestration (R1).

                       Mg₃Si₂O₅(OH)₄ + 3 CO₂ <--> 3 MgCO₃ + 2 SiO₂ + 2 H₂O        (R1)

                                       Serpentine <-->   Magnesite + Quartz

 

Although akin to R1, further geochemical and microscopic analyses indicate the reaction pathway was a three step process.  Step 1, usually minor, resulted in the formation of serpentine and magnesite(1) from any residual olivine that was present.  Talc and magnesite(2) were then produced at the expense of serpentine.  This was then followed by the final step which formed quartz and magnesite(3) at the expense of the talc.  The overall process creates a mineralogical zoned halo around the fluid controlling fractures.  Though complete carbonation (R1) of serpentine involves a 22% volume increase, the talc-forming step which represents 50% of the carbonation potential for serpentine only involves a 1% volume gain.  In situ mineral carbonation within the subsurface may benefit by taking advantage of the talc-forming step where permeability is not significantly destroyed allowing for fresh CO₂-rich fluid to reach unreacted rock.