J.E.T. Channell (Dept. Geological Sciences)
Single-domain (SD) magnetite (in the 30-500 nm size range) is the most
important carrier of remnant magnetization in rocks and sediments.
Larger multi-domain grains carrier a "softer" magnetization that is usually
unstable on geologic timescales. At the low concentrations of magnetite
(and other magnetic minerals) in sediments and rocks, the inter-particle
distances become so large that it is difficult to find and focus on individual
submicron and micron-scale particles using scanning electron microscope
(SEM) or transmission electron microscope (TEM). For this reason,
SEM and TEM studies are usually conducted on magnetic separates produced
by dissolving or separating the non-magnetic matrix (such as calcium
carbonate) using reagents (such as acids). Magnetic separations have the
disadvantage that they are selective. The separation products rarely reflect
the range of magnetic grains present in the sediment or rock sample, and
do not permit observation of the relationship of magnetic particles with
each other, and with the non-magnetic matrix.
In a pilot project conducted during the last two years, we have developed techniques for Fe-mapping the polished surfaces of limestone samples using the high intensity X-ray beam available through the University of Florida partnership in the MR-CAT research module at the Argonne National Laboratory. Synchrotron radiation provides high intensity photons over a wide range of energies that can be focused to narrow (1-5 µm) beam-widths. The high intensity X-ray beam produces fluorescence/absorption that gives a detection limit of ~1 ppm for an element such as Fe. This allows a single 10nm particle to be detected within a 3 µm beam-width.
Determination of chemical composition of Fe anomalies on the map can be based on analysis of three regions of the absorption/energy plot close to the absorption "edge" for Fe. The "pre-edge" provides information on the oxidation state of the absorber, and absorber-ligand bonding, and can therefore be used to estimate the oxidation state of iron. Small shifts in photon energy at the Fe-edge have also been shown to indicate oxidation state. X-ray Absorption Near Edge Structure (XANES) is sensitive to the arrangement of neighbors around the absorber, and can be used as a fingerprint for comparison of unknown samples to standards. The Extended X-ray Absorption Fine Structure (EXAFS) observed at incident energies beyond the XANES region can be compared with computer-generated EXAFS produced from model chemical compositions.
The link between magnetite and probable early bacterial life on Mars dated at ~4 Ga (Thomas-Kleptra et al., 2000) and the implication that bacterial life could have survived inter-planetary transport within Martian meteorite ALH84001 (Weiss et al., 2000), indicates that the detection and imaging of magnetite in terrestrial sediments will become a focus of the search for early life on Earth. X-ray absorption studies can be very sensitive to Fe-bearing particles and can be used to detect sub-micron and micron-scale grains within the (~3µm) X-ray beam. Regions of interest can then be studied with microprobes and SEMs without resorting to magnetic separates, thereby allowing observation of the relationship of these grains to each other and to the non-magnetic matrix.