Analogues to mineral sequestration of CO2: Sources of carbon in magnesite of Attunga Magnesite Quarry, NSW, Australia, a stable isotope study

H.C. Oskierski

Introduction Carbon dioxide sequestration or disposal is an essential component of the international effort to stabilise CO2 emissions to the atmosphere. Of the proposed sequestration schemes, mineral sequestration represents the most geologically stable and environmentally benign method for carbon disposal (Lackner et al. 1995). Mineral carbonation mimics natural silicate weathering processes that bind CO2 in stable carbonate minerals. Ultramafic rocks from ophiolite belts, containing high abundances of magnesia as serpentine and olivine, represent the best potential feedstock for mineral carbonation (Metz et al. 2005). At present, research efforts focus on the development of economically viable and energy efficient processes for large-scale industrial implementation of mineral carbonation. These efforts could be assisted by gaining enhanced understandingand characterisation of the natural carbonation of ultramafic rocks. Earlier studies have shown the outstanding potential of serpentinites of the Great Serpentinite Belt for CO2 sequestration. Based on an RCO2 of 2.46 (the number of tonnes of rock required to sequester one tonne of CO2) and geophysical modelling of part of Great Serpentinite Belt in the northern New South Wales, Davis (2008) concluded that 24 × 10^9 t CO2 could be sequestered, equivalent to 308 y of the total stationary emissions for NSW at 2005 levels. Natural carbonation of the ultramafic rocks of the Great Serpentinite Belt is common and, among others, manifests itself in the development of magnesite deposits and silica carbonate alteration zones (Ashley 1995, 1997). The first step in the understanding of these analogues to mineral sequestration is to identify and trace the reactants. Commonly, cross plots of stable isotopes of carbon (delta13C) and oxygen (delta18O) are used to differentiate between magnesite occurrences and to deduce their sources (Kralik et al. 1989). The sources of carbon cannot always be unequivocally identified, since different processes and mixing can lead to similar, ambiguous carbon and oxygen fingerprints of the magnesite minerals. However, pathways and mechanisms of formation can be constrained if the isotopic fractionations associated with the reaction steps involved are known. In the case of vein deposits of magnesite associated with ultramafic rocks debate has focussed on plant respiration and decay in contrast to metamorphic exhalation as the source of carbon in these deposits (Abu-Jaber and Kimberly 1992). This study seeks to constrain the sources of carbon involved in the carbonation of the ultramafic rocks of the Great Serpentinite Belt that led to the formation of the Attunga magnesite deposit.

Analogues to mineral sequestration of CO2: Sources of carbon in magnesite of Attunga Magnesite Quarry, NSW, Australia, a stable isotope study

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