RUI: Raman Spectroscopy and Stable Isotope Investigations of Graphite in Marble

Mount Holyoke College, South Hadley MA

Investigators

Abstract

Carbon is a unique element, forming the chemical basis for life as we know it. When living things die, their remains are often oxidized or consumed and thereby recycled in the earth's atmosphere, hydrosphere, and biosphere. Under appropriate conditions, some organic matter is trapped in accumulating sedimentary basins, and thus, cycled through the earth's crust and mantle. This carbonaceous material has many significant consequences, such as comprising deposits of coal, petroleum and natural gas. But aside from economically important accumulations of fossil fuels, the fate of small, dispersed bits of organic matter in rocks of the earth's crust can tell us much about ancient sedimentary environments, the history of life, and geochemical processes that operate in crustal environments. This project will investigate the physical and chemical processes that ultimately convert carbonaceous material in sediment into the mineral graphite. This conversion, known as graphitization, involves reorganization of the carbon and loss of other elements, and occurs in response to heat, pressure and deformation during metamorphism. As our understanding of this process improves, so will our ability to use carbonaceous material to better constrain the temperature-pressure-deformation history of geological events, and also to possibly recognize differences in the organic matter originally deposited in ancient sediments. The objectives of this research are to clarify the relationship between metamorphic temperature and the structural state of carbonaceous material (CM) and graphite, and to correlate the conversion of CM to graphite with the exchange of carbon isotopes between graphite and calcite. This project will use laser Raman spectroscopy to characterize the crystallinity of CM and graphite in low- to high-grade marble and compare that for the first time with temperature dependent calcite-graphite isotopic fractionations. Raman analytical techniques and results will be critically evaluated for reproducibility, within-sample variability, across-outcrop variability, preparation methods, and retrograde metamorphism. The fractionation of stable isotopes of carbon between calcite and graphite is temperature dependent and useful as a geothermometer in middle amphibolite to granulite facies marble, however, the isotopic fractionations observed in lower temperature rocks do not agree with theoretically expected fractionations. Different size fractions of CM will be studied isotopically and by laser Raman to relate graphite coarsening with isotopic exchange, and will reveal whether or not prograde isotopic zoning exists in graphite. Comparing the structural state of graphite with the extent of the carbon isotope exchange will help us better understand the isotopic exchange processes and may lead to new tools for unraveling the thermal history of moderate- to low-temperature rocks.