HR: 0800h
AN: MR11A-0884    [Abstracts]
TI: Generation of Methane Via High Pressure Carbonate Reduction
AU: * Scott, H P
EM: hpscott@iusb.edu
AF: Department of Physics and Astronomy, Indiana University South Bend, South Bend, IN 46615 United States
AU: Hemley, R J
EM: hemley@gl.ciw.edu
AF: The Geophysical Laboratory, 5251 Broad Branch Road NW, Washington, DC 20015 United States
AU: Mao, H
EM: h.mao@gl.ciw.edu
AF: The Geophysical Laboratory, 5251 Broad Branch Road NW, Washington, DC 20015 United States
AU: Herschbach, D R
EM: herschbach@chemistry.harvard.edu
AF: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138 United States
AU: Fried, L E
EM: fried1@llnl.gov
AF: Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AU: Howard, W H
EM: howard11@llnl.gov
AF: Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AU: Bastea, S
EM: bastea2@llnl.gov
AF: Physics and Advanced Technologies Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AB: We have designed and conducted experiments to make in situ observations of hydrocarbon formation at conditions relevant to Earth's upper mantle. Specifically, we have used both laser and resistively heated diamond anvil cells to examine an assemblage initially consisting of FeO, CaCO$_{3}$ and H$_{2}$O. We have spanned pressures from 2 to 11 GPa and temperatures from 200 to 1500$^o$C. Synchrotron x-ray diffraction was used to identify solid phases and Raman spectroscopy was used to detect hydrocarbon species. At high pressures and temperatures FeO oxidizes to form Fe$_{3}$O$_{4}$-magnetite. We have found the conditions most productive for methane formation are at pressures less than 5 GPa and temperatures near 500$^o$C, but that methane forms at all conditions of this study. Furthermore, the results are shown to be consistent with multi-phase thermodynamic calculations based on the statistical mechanics of soft particle mixtures. The assemblage FeO-CaCO$_{3}$-H$_{2}$O was studied by previous workers at 5 GPa and 1,500 $^o$C using an apparatus by which run products were analyzed after pressure and temperature quench. Contrary to the previous work, our in situ observations do not indicate the presence of heavier hydrocarbons. However, the observation of methane formation at mantle conditions is significant because it demonstrates the existence of abiogenic pathways for the formation of hydrocarbons in the Earth's interior and suggests that the hydrocarbon budget of the bulk Earth may be larger than conventionally assumed. It is conceivable that heavier hydrocarbons may yet be produced by high pressure carbonate reduction.
DE: 4825 Geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting