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.
The goal of this research project is to improve our understanding of how organic matter in sedimentary rocks is transformed into graphite during metamorphism (such as mountain-building events). This is done using stable isotopes and laser Raman spectroscopy to relate the graphitization process with carbon isotope exchange between coexisting calcite and newly formed graphite. Previous workers have used Raman spectra to estimate metamorphic temperatures of graphite-bearing rocks that were once mudstone and shale (pelite), and other workers have used the carbon isotope ratios in coexisting calcite and graphite to estimate the metamorphic temperatures of graphitic carbonate rocks (marble), but no study has previously compared these two "geothermometers." One important outcome of this research is to show for the first time that these two systems do not agree. It appears that graphite matures at lower temperature in marble than in pelite. Thus, the Raman-based geothermometer requires revision, most likely involving specific application protocols. Furthermore, this research has shown that natural graphite crystal "flakes" yield remarkably different Raman spectra depending on whether one is looking at the large flat crystal faces or a smaller face on the crystal edge. The temperatures calculated for these two spectra (from a single crystal) differ by 170 degrees C. Thus, future applications of this system to geothermometry must take this finding into account. This result also has implications for scientists using Raman spectroscopy to characterize the coarsening of carbon particles in manufacturing. The stable isotope results have two important outcomes. First, application of the calcite-graphite geothermometer to rocks in three different areas covering parts of New York state and southern Ontario, Canada, have improved our knowledge of the metamorphism and mountain-building event that occurred there 1.1 to 1.2 billion years ago (the Grenville orogeny). Second, analyses of graphite of varying sizes within single marble samples have shown that the carbon exchange between the graphitizing organic matter and coexisting calcite is most complete in the coarsest graphite grains and the smallest grains are dominated by the original carbon in organic matter precursors. However, this is only the case for some samples, but not most samples, and is limited to rocks that were exposed to peak temperatures below about 500 degrees C. Some previous workers have suggested that calcite and graphite fail to reach exchange equilibrium with respect to carbon isotopes at temperatures below about 600-650 degrees C. However, because most samples I have examined (from ~500 degree C rocks) do not show different amounts of carbon exchange with graphite grain size, disequilibrium due to the coarsening process during graphitization can be ruled out. Preliminary analyses to date using SIMS (Secondary Ion Mass Spectrometry performed at the University of Wisconsin-Madison Department of Geology & Geophysics) also indicate that graphite in marble is homogeneous within grains and within samples. A secondary outcome of this project has been the research experience and field training opportunities provided to twenty-five female undergraduate geology students at Mount Holyoke College.
