CHARGE CONTRAST IMAGING REVEALS RADIATION-DAMAGE HALOS IN GARNET

Charge contrast imaging (CCI) done using a variable-pressure scanning electron microscope can reveal areas with different densities of crystal defects. CCI has allowed imaging of radiation-damage halos (radiohalos) in pelitic garnets. These radiohalos are not visible optically, or in backscattered electron, secondary electron, or orientation contrast imaging. Radiohalos appear to be developed around small inclusions present in the garnet. Morphology of the radiohalos is consistent with both those seen optically in biotite and those seen in quartz using cathodoluminescence. Radiohalos are generally tens of microns in cross-sectional diameter. Commonly they are concentrically zoned with areas of different degrees of damage which relate to interaction with radioactive decay products of varying energies. If the mineral serving as a source of radioactivity is elongate, the radiohalo commonly has an elliptical cross section. Garnets from several locales show radiohalo development to varying degrees.

Electron backscatter diffraction (EBSD) mapping was preformed on a radiohalo and the area surrounding it to investigate the degree to which the crystal structure may have been disturbed. The crystallographic orientation of the material in the radiohalo area is not affected. However, disturbance of the crystal structure inside the radiohalo relative to outside it is shown in two ways. The degree of pattern misfit of orientations as expressed by the mean angular deviation of the solutions is higher inside the radiohalo. Also, the band slope (another measure of pattern quality) of the EBSD patterns of the garnet in the radiohalo decreased. Both of these are consistent with the crystal structure of the garnet in the radiohalo area having been disturbed by radioactive decay products.