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

从Mg、Ca、Sr稳定同位素看白云石问题的新观点

基本信息

批准号：
395854813
负责人：
Professor Dr. Michael Tatzel
金额：
--
依托单位：
University of California, Santa Cruz (UCSC)
依托单位国家：
德国
项目类别：
Research Fellowships
财政年份：
2018
资助国家：
德国
起止时间：
2017-12-31 至 2019-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/395854813?language=en
关键词：
new view dolomite problem Mg

项目摘要

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

白云石(MgCa(CO3)2)是地球历史上丰富的造岩碳酸盐矿物，但在现代碳酸盐沉积环境中很少形成。地球科学的这一难题也被称为“白云石问题”。白云石一般可分为原生白云岩和次生白云岩，前者是在微生物活动的作用下从水溶液中沉淀出来的，后者是方解石中的镁在沉积后发生置换而形成的。到目前为止，还没有任何结构、化学或同位素标准可以区分这些根本不同的白云石类型。多收集器电感耦合等离子体质谱(MC-ICP-MS)方法的发展首次实现了在白云岩中分别以主量元素和微量元素形式存在的镁和锶的稳定同位素比值的测量，这可能有助于解决白云石问题。与胞外聚合物(EPS)有关的白云岩中的镁(Bontognali等人，2014)和锶(Paytan博士的初步研究)的新证据表明，在EPS的存在下，原生微生物介导的白云岩中的镁和锶稳定同位素可能被动力学分馏，或者达到特定的平衡同位素分馏因子。为了解决白云岩的成因问题，我们提出了根据稳定的镁、钙、锶同位素比值来确定白云石的形成方式(原生与次生)。为了解决白云岩的成因问题，我想通过降水实验和地质记录对白云石进行稳定的金属同位素分析，研究微生物沉淀白云岩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：第一，测定同位素分馏系数α26/24 mg(白云岩溶液)、α44/40Ca(白云石溶液)和α88/86Sr(白云石溶液)，以及来自巴西现代泻湖‘Lagoa Vermelha’的白云石和稀有的天然原生白云岩中的元素浓度。其次，分析太古界和新近系白云岩的镁、钙、锶、碳、氧稳定同位素比值和化学成分，以确定它们的形成模式。我希望确定原生白云岩特有的同位素分馏因子，以区分次生白云岩，从而得出整个地球历史上白云岩形成模式的变化。由于原始白云石的沉淀依赖于(氧化还原依赖)微生物生境的存在，因此在地球海水和大气的两阶段氧化过程中，白云石形成模式的长期变化是可以预见的。