Mg-Isotope Signatures for Tracing of Natural Carbonation Reactions

Authors: 
Oskierski, H. C. - Presenter, Murdoch University
Beinlich, A., Curtin University
Altarawneh, M., Murdoch University
Dlugogorski, B. Z., Murdoch University
Mg-carbonates are both economically and fundamentally significant as they occur i) as valuable deposits of high-purity magnesite ore and in association with epithermal gold mineralisation, ii) as important reservoirs in the global carbon and magnesium cycles and iii) as products of natural carbonation reactions, capable of storing large amounts of CO2. Textural observations, ranging from field- to microscale, and stable isotope signatures of carbon and oxygen, which enable constraining potential fluid and carbon sources, commonly serve to evaluate the formation conditions of Mg-carbonates.

Mg-isotopes undergo fractionation during low temperature surface processes such as weathering and carbonation of ultramafic rocks and thus bear great potential to enhance our understanding of the formation of Mg-carbonate minerals. Despite their relevance for a wide range of geological processes, studies focussing specifically on the Mg-isotope signatures of magnesite (MgCO3) are rare1-3.

Our data demonstrates that, the Mg-isotopic composition of serpentinites from Eastern and Western Australia concurs with other mafic and ultramafic rocks worldwide and falls close to the composition of bulk silicate Earth4. The δ26Mg of weathered serpentinite relates inversely with MgO content but increases with the degree of weathering as well as clay mineral content, indicating that the neo-formation of Mg-bearing clay minerals controls Mg-isotope fractionation during weathering of serpentinites. The clay minerals retain heavy 26Mg, while predominantly releasing light 24Mg into the weathering fluids, consistent with previous observations of continental run-off being isotopically lighter than the drained lithologies5,6.

Both low and high-temperature magnesites display lower δ26Mg than the weathering fluids, reflecting preferential incorporation of 24Mg into magnesite, in accordance with the bonding environment of Mg in the magnesite structure7. Despite the common, relatively homogeneous source of Mg from ultramafic rocks, we detect large variations in the Mg-isotopic signatures of a range of Australian magnesite deposits. Our study confirms both mineralogy and precipitation temperature as primary factors controlling Mg-isotope signatures of Mg-carbonates but also highlights potential influence of aqueous Mg-speciation and fluid pH, as well as kinetic effects during precipitation and diagenetic overprinting on the observed ratios of Mg-isotopes8-10.

References:

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