HR: 0830h
AN: B51C-0980    [PDF]
TI: Pyrite Framboids: A Possible Biosignature For the Study of Ancient Sediments
AU: * Schelble, R T
EM: rachels@usc.edu
AF: University of Southern California, 3651 Trousedale Parkway, Los Angeles, CA 90089 United States
AU: Popa, R
EM: rpopa@usc.edu
AF: University of Southern California, 3651 Trousedale Parkway, Los Angeles, CA 90089 United States
AU: Douglas, S
EM: susanne.douglas@jpl.nasa.gov
AF: NASA Jet Propulsion Laboratory, 4800 Oak Grove Dr. Mail Stop 183-301, Pasadena, CA 91801 United States
AU: Nealson, K H
EM: knealson@usc.edu
AF: University of Southern California, 3651 Trousedale Parkway, Los Angeles, CA 90089 United States
AB: Framboids are 0.5-100 $\mu$m-sized, raspberry-like, spheroidal accumulations of 10-10$^{9}$ microcrystallites. These structures are predominantly composed of pyrite (FeS$_{2}$), although other minerals such as greigite (Fe$_{3}$S$_{4}$), magnetite (Fe$_{3}$O$_{4}$), and limonite (FeO$^{.}$OH$^{.}${\it n}H$_{2}$O) have also been identified. Modern framboids are present in a variety of natural environments (soils, sediments, microbial mats), and can also be produced artificially in the laboratory. Even though numerous cases of framboids have been cited from the geological record, little is known about their mechanisms of formation, and biogenicity. Understanding the effects of the conditions of formation upon framboid characteristics from modern environments (e.g. size distribution, microtexture, composition, and mineralogy) will allow us to better understand what pyrite framboids can tell us about sediments from the geological record. The purpose of this project was to determine whether pyrite framboids can be used as a biosignature within ancient sediments. Microbial mats rich in pyrite framboids were collected from the Buncar Springs in Mangalia aquifer (Romania). The pyrite framboid rich microbial mats are formed underground at the anaerobic water-gas (90-95% methane, 5-10% CO$_{2}$) interface. Neoproterozoic (750-600 Ma) manganese carbonate deposits containing fossilized pyrite framboids were collected from a mine near the city of Ming Lo, Hunan Province, China. Synthetic framboids were made in the laboratory by a modification of the protocol provided by Sweeney and Kaplan (1973). The morphology of framboids formed within modern microbial mats, within ancient sediments, and those synthesized abiotically in the laboratory were compared, and then correlated with their biogenicity. Images were obtained using environmental scanning electron microscopy and the microtextures were analyzed using shape-analysis tools providing information about the overall size distribution of framboids, number of microcrystallites per unit volume, and size distribution of microcrystallites. The results can be used to make inferences about the abiotic genetic conditions and on the origin of framboids from the geological record. Sweeney, R.E., and Kaplan, I.R. (1973). Pyrite Framboid Formation: Laboratory Synthesis and Marine Sediments. Economic Geology, 68, 618-634.
DE: 1615 Biogeochemical processes (4805)
DE: 3625 Descriptive mineralogy
DE: 4802 Anoxic environments
DE: 4840 Microbiology
DE: 5112 Microstructure
SC: Biogeosciences [B]
MN: 2003 Fall Meeting