NEUTRON TOMOGRAPHY IMAGES OF MICROBIAL STRUCTURES IN ARCHEAN CARBONATES

Neutron imaging is a useful, non-destructive tool for the study of microbial structures in carbonates and silicates. A neutron beam interacts with atomic nuclei and is attenuated by scattering or absorption. The process of neutron tomography takes 2-D images of neutron transects through a sample and uses them to reconstruct 3-D volumes, which is the same type of reconstruction process done in X-ray CT. Neutron imaging is useful for studying microbial textures in rocks because H, C, O, and Si have varied neutron attenuation coefficients. Hydrogen is much more attenuating than other elements found in carbonates and cherts, for example, and its distribution can be mapped with neutron imaging. Areas in samples that are rich in organics, which contain H, show up well against bordering areas of lower attenuation. However, neutron imaging techniques need to be tuned to geological samples due to complexities in neutron-sample interactions. Beam hardening, a problem when the neutron beam used for imaging has neutrons with varying energies, needs to be corrected empirically by measuring attenuation of pure samples of varying thickness. Also, image artifacts due to instrumentation, noise and scatter need to be removed. Images of pure calcite wedges and Archean carbonates, containing microbial structures demonstrate the usefulness of the technique. Several cores containing Archean microbialites have been imaged and the resulting reconstructions show distinct areas of high attenuation. The samples contain highly attenuating mat-like structures. These reconstructed volumes, and others to come, produced by this technique will be very useful in the study of the textures contained in rocks. This technique also has applications for life detection studies that will be pursued. Areas in unknown samples high in organics can be found without damage or contamination (provided the samples are collected in a container to neutrons, such as aluminum). With sample return from Mars becoming more of a possibility, this technique could pinpoint areas of study within Martian samples that could contain traces of extraterrestrial life.