A new view on the dolomite problem from Mg, Ca and Sr stable isotopes

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Dolomite (MgCa(CO3)2) is an abundant rock-forming carbonate mineral throughout Earth history; however, it rarely forms in modern carbonate depositional environments. This conundrum of Earth Sciences is also known as the “dolomite problem”. Dolomite can in general be divided into primary dolomite – which precipitates from aqueous solutions mediated by microbial activity, and secondary dolomite – which forms diagenetically by post depositional replacement of Ca by Mg in calcite. To date, no structural, chemical, or isotopic criterion exists that allows distinguishing these fundamentally different types of dolomite. Methodological developments in multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) allow for the first time the measurement of stable isotope ratios of Mg and Sr that occur as major- and trace elements in dolomite, respectively and might contribute to solving the dolomite problem.New evidence for Mg- (Bontognali et al., 2014) and Sr enrichment (pilot study by Dr. Paytan) in dolomite associated with exopolymeric substances (EPS) suggest that Mg and Sr stable isotopes in primary, microbially mediated dolomite may be kinetically fractionated or that specific equilibrium isotope fractionation factors are attained in the presence of EPS. We propose that the mode of dolomite formation (primary vs. secondary) can be determined based on stable isotope ratios of Mg, Ca and Sr. To contribute solving the dolomite problem, I want to investigate if the conditions for microbial dolomite precipitation have changed based on stable metal isotope analyses of dolomite from precipitation experiments and the geological record. In this project, I pursue two principle objectives: Firstly, the determination of isotope fractionation factors α26/24Mg(dolomite-solution), α44/40Ca(dolomite-solution), and α88/86Sr(dolomite-solution) and element concentrations in dolomite from precipitation experiments with EPS, and in rare, natural primary dolomite from the modern lagoon ‘Lagoa Vermelha’ in Brazil. Secondly, the analysis of Mg, Ca, Sr, C, and O stable isotope ratios and chemical composition of Archean to Neogene dolomite to determine their mode of formation.I expect to determine isotope fractionation factors that are specific to primary dolomite and that these allow distinguishing secondary dolomite and hence to deriving changes in the modes of dolomite formation throughout Earth history. As primary dolomite precipitation depends on the presence of (redox-dependent) microbial habitats, secular changes in the mode of dolomite formation are anticipated during the Earth’s two-stage oxygenation of seawater and atmosphere.Bontognali, T.R.R., Mckenzie, J.A., Warthmann, R.J., and Vasconcelos, C., 2014, Microbially influenced formation of Mg-calcite and Ca-dolomite in the presence of exopolymeric substances produced by sulphate-reducing bacteria: Terra Nova, v. 26, no. 1, p. 72–77