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

白云石（MgCa(CO3)2）是贯穿地球历史的丰富的碳酸盐岩造岩矿物；但在现代碳酸盐岩沉积环境中很少形成。这个地球科学难题也被称为“白云岩问题”。白云岩一般可分为原生白云岩和次生白云岩。原生白云岩是由微生物活动介导的水溶液沉淀而成，次生白云岩是由方解石中的钙在沉积后被镁取代而形成的成岩作用。迄今为止，没有任何结构、化学或同位素标准可以区分这些根本不同类型的白云岩。多收集器电感耦合等离子体质谱（MC-ICP-MS）方法的发展首次允许测量白云岩中分别作为主要元素和微量元素的Mg和Sr的稳定同位素比率，并可能有助于解决白云岩问题。新的证据表明，与外聚合物物质（EPS）相关的白云岩中Mg- (Bontognali et al., 2014)和Sr富集（Paytan博士的初步研究）表明，原生微生物介导的白云岩中的Mg和Sr稳定同位素可能会被动力学分馏，或者在EPS存在的情况下获得特定的平衡同位素分馏因子。我们提出可以根据Mg、Ca和sr的稳定同位素比值来确定白云岩的形成模式（原生与次生）。为了有助于解决白云岩问题，我想根据沉淀实验和地质记录对白云岩的稳定金属同位素分析来研究微生物白云岩沉淀的条件是否发生了变化。在这个项目中，我追求两个主要目标：首先，测定同位素分馏因子α26/24Mg（白云岩溶液），α44/40Ca（白云岩溶液），α88/86Sr（白云岩溶液）和元素浓度，这些元素分别来自EPS沉淀实验的白云岩，以及来自巴西“Lagoa Vermelha”现代泻湖的稀有天然原生白云岩。其次，分析太古宙—新近系白云岩的Mg、Ca、Sr、C、O稳定同位素比值及化学成分，确定其形成方式；我希望确定原生白云岩特有的同位素分馏因子，这些因子可以区分次生白云岩，从而推导出整个地球历史上白云岩形成模式的变化。由于原生白云岩降水依赖于（氧化还原依赖的）微生物栖息地的存在，因此在地球海水和大气的两阶段氧化过程中，白云岩形成模式的长期变化是可以预测的。陈志强，陈志强，陈志强，陈志强，2014，硫酸盐还原菌对镁方解石和钙白云石形成的微生物影响，环境科学进展，vol . 26, no. 1。1，第72-77页

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