ANNEALING CHARACTERISTICS OF RADIATION DAMAGE HALOS IN QUARTZ AS REVEALED BY CATHODOLUMINESCENCE MICROSCOPY

Quartzite and quartz sandstone commonly contain uranium bearing zircon inclusions surrounded by radiation damage halos observable using cathodoluminescence microscopy. Preservation of radiation damage is time and temperature sensitive and halos are annealed under mid-crustal geologic conditions. Determining the temperature and pattern of quartz radiation damage annealing informs our understanding of the thermal history of a rock, and may lead to development of new thermometric methods techniques.

In order to determine this annealing temperature, we collected quartzite samples from the Cambrian Erwin Formation. The Erwin formation is a white orthoquartzite with scolithus and diplocraterian trace fossils, the presence of these fossils and the absence of recrystallization fabrics, suggests that these rocks have not experience significant heating since deposition.

We cut samples into two centimeter square billets which we polished and heated to varying temperatures and times from 200ºC to 1000ºC and 20-200 hours. The polished surfaces were then inspected for radiation damage halos around zircon inclusions using a chromaCL cathodoluminescence detector to characterize any annealing effects. This series of steps allows us to bracket the temperature and time at which radiation damage halos are annealed.

The observations we made using cathodoluminescence show that two types of halos exist in quartz, a red halo, and a blue halo. Previous authors have determined that the blue halo is caused by a silica deficiency resulting from alpha decay and is visible in reflected light cathodoluminescence in samples heated to between 400ºC and 600ºC. The cause of the red halo is unknown and is observed in in reflected light cathodoluminescence in samples not heated above 800ºC. Previous studies have shown that not until above 800ºC is radiation damage in quartz completely annealed; however, our results differ, and we observe that radiation damage, of both types, is completely annealed at 600ºC despite using identical analytical parameters as previous studies. Our results show that there may be more parameters that control annealing of radiation damage in quartz grains than previously described, such as total fluence, or chemical/physical sample characteristics.