Does Magnetite Oxygen Isotopes reveal a Biosignature?
Abstract
Magnetite [FeIII_2FeIIO_4] nanoparticles have been found in the ALH 84001 meteorite. Their possible biogenic origin has generated a strong debate which emphasizes that purely inorganic magnetites cannot easily differentiated from those made by bacteria. Oxygen isotopic composition of magnetite could be potentially useful for characterizing biogenic magnetites in the environment and for the development of a magnetite / phosphate geothermometers (Mandernack et al., 1999). Little is known, however, about the oxygen isotopic compositions of inorganically produced nanometer-sized magnetites. Inorganic magnetites have been synthesized under controlled chemical affinity conditions at oxygen fugacity close to 0, ionic strength of 0.2 M and high solution saturation state (> 100) in order to determine if oxygen isotopes can be used as a proxy of bacterial signature. Total dissolved iron of stoechiometric ratio (i.e. FeII / FeIII = 0.5) and temperature were varied to identify the role of these variables on the kinetic rate and on the oxygen isotope fractionation. Under our experimental conditions, magnetite particles of an average dimension of 8 nm are formed only when [Fetot] is higher than 9 mM while in lower dissolved iron concentration, only goethite [α-FeIIIO(OH)] particles are formed. Furthermore, when temperature was increased, higher iron concentration were necessary to form magnetite. Oxygen isotopes fractionation, 10^3ln αm-w, is important for low iron concentration and stabilizes around 0 to 1 ppm for [Fetot] >= 30 mM. Since biotic magnetites have similar oxygen isotope signature (Mandernack et al., 1999) to our abiotic magnetite particles, no biosignature can be observed and we propose that bacteria may simply produce an increase of solution saturation state in the magnetosomes that, in turn, control the precipitation of magnetite nanoparticles in surface conditions. Mandernack K. W. et al. (1999), Science 285, 1892-1896.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....6491F