SGER - Genesis and Diagenesis of an Enigmatic Precambrian Carbonate cement: an Investigation using Microanalytical and Experimental Techniques

项目摘要智能优点：磨牙牙(MT)是一种神秘的碳酸盐织物，其特点是各种形状的空隙填充着均匀、均匀的微晶石。到目前为止，研究主要集中在破译空洞形成的机制和描绘MT在时间和空间上的分布。然而，MT的保存与MT微石的沉淀密切相关，我们对此知之甚少。MT微晶石由均匀、均匀、不互锁的晶体组成，在阴极发光(CL)下显示出暗发光、近球形核心(直径5-7m)和发光过度生长。中元古代和新元古代早期(1600-750 Ma)普遍存在有利的降水条件，其中MT微晶石占单层的90%，保存碳酸盐(体积相当于古生代骨骼颗粒)的5%-25%。该项目的目的是研究一种不寻常的、但具有重要体积意义的碳酸盐形态，被称为磨牙牙(MT)微晶石，它在中元古代和新元古代早期的广泛存在表明，碳酸盐沉积与海洋环境的大范围地球化学演化之间存在联系。这项建议将解决三个问题：(1)这种不寻常的碳酸盐微石的成核和生长机制是什么？(2)以前提出的这种微石的原始矿物学是球状陨石吗？(3)这种不同寻常的碳酸盐形态能告诉我们关于元古界海洋系统组成的什么信息？不幸的是，较小的晶体尺寸阻碍了许多传统技术在进一步研究微石来源方面的使用。我提出了一种多学科方法，其中高分辨率化学发光和扫描电子显微镜成像将用于定义形态端元，重建晶体尺寸分布，并解释成核和生长机制；激光拉曼光谱将用于记录晶格结构，以帮助约束矿物学变化；高分辨率SIMS微量元素成像、拉曼光谱和晶体生长实验将用于约束初始流体成分和沉淀条件。最终，破译MT微晶石的成因将有助于识别形成的化学环境，并有助于了解地球生物圈长期化学变化的沉积影响。更广泛的影响：该项目旨在促进发现和理解，同时促进教学、培训和学习，方法是扩大田纳西大学基于碳酸盐的研究的广度，为本科生增加研究机会，让学生直接参与科学研究过程，鼓励本科生在研究生和教职员工的帮助和指导下进行科学活动，并鼓励学生参与地区和国家会议。目前，我有3名学生排队参加这个项目[1名硕士(男性)和2名学士(女性)；其中2名是成家后重返校园的非传统学生]。该项目还将加强德克萨斯大学的研究和教育基础设施，支持扩大多用户设施的使用，并在大学内部(地质-化学)和与橡树岭国家实验室建立跨学科合作。最后，该项目旨在通过继续与西乔治亚州立大学进行基于研究和设备的合作，扩大代表性不足群体的参与，西乔治亚州立大学是一所非授予博士学位的机构。这样的合作提高了学生对地球科学机会的认识，并加强了德克萨斯大学吸引新研究生的能力。

PROJECT SUMMARY Intellectual Merit: Molar-tooth (MT) is an enigmatic carbonate fabric marked by variously shaped voids filled with a uniform, equant microspar. To date, research has focused largely on deciphering mechanisms of void formation and delineating MT distribution in time and space. MT preservation, however, is intimately linked to precipitation of MT microspar, about which we know little. MT microspar consists of uniform, equant, non-interlocking crystals that under cathodoluminescence (CL) show dully-luminescent, subspherical cores (5-7 m diameter) and luminescent overgrowths. Conditions favoring precipitation were common in the Mesoproterozoic and early Neoproterozoic (1600-750 Ma), where MT microspar comprises 90% of individual beds and 5-25% of preserved carbonate (volumetrically equivalent to Paleozoic skeletal grains). The objective of this project is to investigate an unusual, yet volumetrically significant, morphology of carbonate, termed molar-tooth (MT) microspar, whose widespread occurrence in the Mesoproterozoic and early Neoproterozoic suggests linkages between carbonate precipitation and the broad-scale geochemical evolution of marine environments. This proposal will address three questions: (1) What are the nucleation and growth mechanisms of this unusual carbonate microspar? (2) Was vaterite the original mineralogy of this microspar, as has been previously proposed? and (3) What can this unusual carbonate morphology tell us about the composition of Proterozoic marine systems? Unfortunately, small crystal sizes inhibit use of many traditional techniques in further examining microspar origins. I propose a multidisciplinary approach wherein high-resolution CL and SEM imaging will be used to define morphologic endmembers, reconstruct crystal size distributions, and interpret nucleation and growth mechanisms; laser-Raman spectroscopy will be used to document lattice structure to help constrain mineralogical change; and combined high-resolution SIMS trace element imaging, Raman spectroscopy, and crystal growth experiments will be used to constrain initial fluid compositions and precipitation conditions. Ultimately, deciphering the genesis of MT microspar will help identify chemical environments of formation and will aid in understanding the sedimentary impact of long-term chemical changes in the Earth.s biosphere. Broader Impacts: This project aims to advance discovery and understanding while promoting teaching, training, and learning by expanding the breadth of carbonate-based research at the University of Tennessee, adding research opportunities for undergraduate students, involving students directly in the process of scientific inquiry, encouraging scientific activity in which undergraduates will assist and be mentored by graduate students and faculty, and encouraging student participation in regional and national meetings. Currently, I have 3 students lined up for this project [1 M.S. (male) and 2 B.S. (female); 2 of them non-traditional students returning to school after starting families]. This project will also enhance the research and education infrastructure at the UT by supporting initiatives to expand use of multi-user facilities and establish interdisciplinary collaboration within the University (Geology-Chemistry) and with Oak Ridge National Labs. Finally, the project aims to broaden participation of under-represented groups through continued research- and equipment-based collaboration with the State University of West Georgia, a non-Ph.D. granting institution. Such collaboration raises student awareness of opportunities in the Earth Sciences and strengthens UT.s ability to attract new graduate students.