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

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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.