MULTI-SCALE MAGNETIC FOOTPRINT OF SERPENTINITE CARBONATION – IMPLICATIONS FOR GEOPHYSICAL MONITORING OF IN SITU GEOLOGICAL CO2 SEQUESTRATION

Earth’s climate change is intrincically linked to the CO2 cycle. Toward better understanding of the feedback system between atmosphere and lithosphere, and ultimately toward much effective approach for long-term atmospheric CO2 mitigation, sequestrating CO2 via in situ mineral carbonation, which mimics naturally occurring CO2 alteration of mantle peridotite, has been suggested. Although many field and laboratory studies have focused on understanding reaction pathways to upscale CO2 uptake from lab to field scale, the development of tools and techniques capable of characterizing the reaction progress in situ has received little attention. Here we present the first case study aiming at establishing a geophysical monitoring scheme of the reaction progress of in situ CO2 sequestration in ultramafic formations. Distinct magnetic property changes, related to mineral replacement reactions along the reaction path of ultramafic rock carbonation, are investigated by integrating multi scale (km-m-µm) magnetic data and field and petrological observations from the Linnajavri ophiolite, Norway. At this site, carbonation was driven by infiltration of a CO2-bearing fluid along steep faults in the serpentinite at ~275 °C and formed sharp reaction fronts, representing a natural analog for a CO2 storage formation. With increasing carbonation, serpentinite is first altered to a magnesite-talc rock (soapstone) then to a magnesite-quartz rock (listvenite) with ~17 wt% and ~30 wt% bulk rock CO2, respectively. Our results show that at all scales, magnetic signal strength increases from serpentinite to soapstone, then decreases from soapstone to listvenite. Thin section petrography reveals that the Fe released from breakdown of serpentine forms fine grained magnetite in the soapstone. In contrast, under listvenite forming conditions Fe-enriched carbonate grows at the expense of Fe-oxide phases. This linear relation between the progression of carbonation and changes in the abundance of magnetic carrier minerals is reflected in magnetic field strength changes across alteration fronts. We propose that magnetic profiling in time and space can be used as a basis for tracking the extent and degree of in situ carbonation of ultramafic formation, thereby a useful monitoring tool for CO2 sequestration programs.