October 2014 LIP of the Month

The CO₂ sequestration potential of the ultramafic portions of Large Igneous Provinces

Siobhan A. Wilson and Simon M. Jowitt

School of Earth, Atmosphere and Environment, Monash University, Wellington Road, Clayton, Melbourne, VIC 3800, Australia

Email: sasha.wilson@monash.edu; simon.jowitt@monash.edu

Carbon mineralization in mafic and ultramafic rocks

Carbon mineralization, the storage of CO₂ within the crystal structures of carbonate minerals, is recognised as one of the safest and most effective methods for controlling the rising concentrations of atmospheric greenhouse gases that are associated with anthropogenic climate change (Seifritz, 1990; Lackner et al., 1995; Lackner, 2003). Unlike traditional Carbon Capture and Storage (CCS), which relies primarily upon stratigraphic trapping of injected supercritical CO₂ in sedimentary rock formations, carbon mineralization technologies employ direct production of carbonate minerals to trap and store CO₂. Sequestration of CO₂ within carbonate minerals is typically achieved by reaction of this greenhouse gas with Mg- and Ca-silicate and hydroxide minerals such as forsterite, serpentine minerals and brucite. The lower reactivity of pyroxenes and other highly polymerized silicate minerals limits the utility of intermediate to felsic rocks as feedstocks for carbon mineralization. However, abundant natural deposits of mafic and ultramafic rocks represent an important resource for managing anthropogenic greenhouse gas emissions.

Carbon mineralization technologies (recently reviewed by Power et al., 2013) are focused on (1) in situ strategies that rely upon injection of CO₂ into mafic and ultramafic geological formations and (2) ex situ technologies that require extraction of ultramafic minerals via mining (or generation of alkaline waste materials by industry). Ex situ carbonation includes carbonation in high-temperature and high-pressure reactors to capture industrial CO₂ emissions as well as enhanced weathering of mineral wastes generated by mining and other industries for direct capture of CO₂ from the atmosphere or industrial point sources.

Here, we discuss the potential for natural and enhanced CO₂ sequestration in Large Igneous Provinces (LIPs), with a focus on the ultramafic portions of LIPs. Mafic rocks are an order of magnitude more common at Earth’s surface than ultramafic rocks and, although they carbonate more slowly, they represent a substantial resource for CO₂ sequestration within LIPs. Trapping and mineralizing CO₂ either within basalts or in basalt-capped reservoirs, including continental flood basalts (CFBs) formed during mafic-ultramafic LIP events (e.g. McGrail et al., 2006), has the potential to offset anthropogenic CO₂ emissions. For example, Matter and Kelemen (2009) and Gislason et al. (2010) report that basalts of the Columbia River LIP have the potential to host large quantities of CO₂ (between 36 and 148 Gt), primarily within low permeability basalts of the LIP that are vesicular and brecciated. The technology required to carbonate basalts in situ (i.e., without extracting basalt for use in reactors) is already being developed and tested. The CarbFix pilot project is now injecting CO₂ charged waters directly into basaltic rocks in southwest Iceland (e.g., Gislason et al., 2010; www.or.is/en/projects/carbfix) and the Big Sky Carbon Sequestration Partnership has been injecting CO₂ into the Columbia River Basalt near Wallula, WA, USA (http://www.bigskyco2.org/research/geologic/basaltproject).

Ultramafic rocks are ideal feedstocks for carbon mineralization because they (1) contain a high proportion of reactive forsterite and serpentine (>80 wt.% in many cases) and (2) produce durable Mg-carbonate minerals as reaction products. Recent research has demonstrated significant sequestration of atmospheric CO₂ by ultramafic rocks and particularly by mineral wastes associated with ore deposits within ophiolite sequences (e.g. Wilson et al., 2009; Power et al., 2009, 2014; Pronost et al., 2012; Beinlich and Austrheim, 2012; Oskierski et al., 2013) and komatiitic lavas within Archean LIPs (Wilson et al., 2014). Carbonation of Mg-silicate minerals in ophiolitic outcrop (and basalt as well) is thermodynamically favoured and proceeds via reaction with atmospheric CO₂ dissolved in rainwater and alkaline spring waters. The rate of CO₂ sequestration is readily enhanced, by over two orders of magnitude, in the finely pulverised mineral wastes generated by ultramafic-hosted ore deposits (Wilson et al., 2009; Power et al., 2013).

The capacity of ultramafic rocks to fix CO₂ is significant. Kelemen and Matter (2008) propose a strategy for in situ carbonation of ophiolites whereby buried ultramafic rocks are accessed by drilling, hydraulic fracture and injection of supercritical CO₂. They suggest that total carbonation of peridotite within the Semail ophiolite of Oman could sequester 30 trillion tons of CO₂, indicating that although total carbonation is highly unlikely, ophiolitic rocks could sequester significant volumes of CO₂. For perspective, current global CO₂ emissions are estimated at 38 Gt per annum, with 76% of emissions resulting from combustion of fossil fuels (IPCC, 2007). Therefore, complete carbonation of the ultramafic rocks within the Semail ophiolite alone would effectively offset global CO₂ emissions for nearly 800 years. However, despite the extensive research that has been undertaken on the CO₂ sequestration potential of ophiolites and CFBs, comparatively little has been done on the CO₂ sequestration potential of the ultramafic portions of LIPs.

LIPs as a resource for reactive ultramafic rocks

The magmatism that generates LIPs can produce large volumes of ultramafic rocks, although the volumes of ultramafic rock within individual LIPs vary significantly (e.g., Ernst, 2014). For instance, the Siberian Trap, Deccan Trap and Columbia River LIPs are dominated by CFBs, whereas the Bushveld LIP, which includes the world’s largest mafic to ultramafic intrusive complex, contains significant amounts of ultramafic rock. Importantly, even CFB-dominated LIPs can contain both ultramafic intrusions and ultramafic (i.e. picritic) basalts. This is exemplified by the Muskox intrusion associated with the 1.27 Ga Mackenzie LIP of northern Canada (Fig. 1), which is dominated by a ~2,400-km long dyke swarm. The Muskox intrusion itself, although comprising a relatively small volume of the LIP, is ~125-km long, may extend another 250 km under cover (e.g. Barnes and Francis, 1995), thins from a width of 11 km to 100 m and is divided into feeder, marginal zones, a central layered series and an upper border group, with the first three of these subdivisions being dominated by or containing ultramafic rocks (e.g. Smith and Kapp, 1963).

Other examples of LIPs that contain significant volumes of ultramafic rocks include the Thompson area of the circum-Superior LIP and various Archean LIPs or LIP analogues (e,g., Ernst, 2014). Importantly, Archean LIPs, such as the magmatism within the Eastern Goldfields Superterrane of the Yilgarn Craton of Western Australia (i.e. the Eastern Yilgarn LIP), are typically associated with voluminous ultramafic komatiites, as well as often hosting significant mafic–ultramafic layered intrusions, such as the ~30,000 km³ Windimurra Complex of Western Australia (Ivanic et al., 2010, 2012). Many komatiite-hosted ore deposits are currently being mined in the Yilgarn Craton, including the Mount Keith Nickel Mine in the Agnew–Wiluna Greenstone Belt (Fig. 2). A recent study demonstrates that, unbeknownst to the mine operator, Mount Keith has been offsetting 11% of its annual greenhouse gas emissions via weathering and carbonation of its mine tailings (Wilson et al., 2014). Changes to tailings management and ore processing practises could be made to further reduce – or completely offset – the emissions produced by Mount Keith and many other mines (Harrison et al., 2013; Wilson et al., 2014). The CO₂ sequestration potential of LIP intrusions is also being actively investigated, with research currently underway to make use of pyroxene-rich mafic tailings from PGM mines in the Bushveld LIP as a feedstock for industrial carbonation reactors (e.g., Meyer et al., 2014).

If and when carbon mineralization technologies become widely implemented, it will become important to accurately determine the amount ultramafic rocks suitable for CO₂ sequestration that are present in LIPs as well as ophiolites. The quantification of the abundance of ultramafic rocks within a given LIP ranges from good (e.g. in the case of the Bushveld LIP, which is dominated by a single, gigantic, well-studied and relatively uniform layered intrusion) to poor. Nevertheless, the level of quantification is improving; this is demonstrated by recent research within the Wrangellia LIP, where geophysical 3D modelling of a ~7 x ~5 km area of the Amphitheater Mountains in Alaska identified 2,000 km³ of ultramafic rocks in the subsurface (to a depth of 4 km; Glen et al., 2011). The results of the latter study indicate that research primarily focused on Ni-Cu-PGE exploration can also feed into determining the CO₂ sequestration potential of the ultramafic portions of LIP events. Furthermore, even in CFB-dominated LIPs, the ultramafic rocks that are most valuable for CO₂ sequestration represent volumetrically significant portions of the rocks formed during these events. For example, the Wrangellia LIP hosts an estimated ~3,500 km² of ultramafic rock within 4 km of the Earth’s surface versus a currently exposed area of ~27,000 km² (Greene et al., 2010) and a total estimated volume of ~1 million km³ (Ernst and Buchan, 2001). All of this demonstrates that even CFB-dominated LIP events can produce voluminous ultramafic rocks of significant value for sequestration of CO₂.

Conclusions

Although the basaltic portions of LIPs have long been recognised as a resource for CO₂ sequestration via carbon mineralization, the CO₂ sequestration potential of the ultramafic components of these LIP events remains understudied. Archean LIPs, komatiite-dominated sections of LIPs (e.g. the Thompson Belt), and layered intrusion-dominated LIPs (e.g., the Bushveld LIP) in particular represent a relatively unexplored and untapped potential resource for CO₂ sequestration. Thus, increased efforts should be made to estimate the size of this resource and to take better advantage of the CO₂ sequestration potential of these and many other LIPs.

References