Research Training Program

Dawn Stodden
University of Montana
Western Dillon, Montana

William Melson, Ph.D.
Supervising Scientist
Department of Mineral Science

"Working at the Smithsonian under the tutelage of brilliant scientists has given me a rare and exciting opportunity to experience scientific research while still an undergraduate."

The Assembly of North America: Using Garnet Zoning to Decipher the Geologic History of the Potomac Terrane

The tectonic evolution of the Appalachian piedmont is a topic of much debate. To decipher this evolution, small pieces of a large puzzle must be put together. In this study, the Sykesville Formation in the easternmost Potomac Terrain was examined. The Potomac terrain is a fragment of continental crust that stuck on to the North American continent probably over 400 million years ago. These studies of the Sykesville formation included structural mapping, precursory rock analysis, and chemical analyses of garnets and other minerals, all indicators of the geologic history of the rocks. Through the structural mapping it was noted that a depositional trend follows the surrounding terrain. In the study area of the Sykesville Formation, there are many quartz structures. These structures appear to be beds or veins. To decipher which they are, a thin section of quartz was analyzed using cathodoluminescence (glow induced by bombardment with electrons). With this analysis, relict sand grains were discovered preserved in the quartz (beds), which has shown some evidence supporting the idea the quartz is depositional in nature. The final part of this project was to examine garnets using electron microprobe analysis, a method in which x-rays are generated by a minute, 2-micron electron beam, impinge on a mineral grain and analyze it for elemental composition. Chemical zoning in garnets acts as a record of the growth history of the garnet, as well as the surrounding rocks. With the microprobe, several X-ray maps were made of some garnets in the Sykesville formation. These garnets have shown that this formation was, most likely, only metamorphosed once after it was deposited. Future analysis of the chemical composition of these garnets will reveal more about the pressures and temperatures that the garnets reached through metamorphism during the ancient collision. With the data compiled in this study, more work to decipher a more precise history of the Sykesville Formation will be possible.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-9820303.

Letter of Gratitude