VETTING UNCONVENTIONAL MINERALS FOR THERMOCHRONOLOGY

The development of new minerals for use in thermochronology has the benefit of being able to date rock types that do not contain traditionally used minerals (e.g. zircon, apatite), as well as fill in gaps that currently exist while reconstructing tectonic cooling histories, especially in mid-temperature ranges (200-400C). As new minerals, such as magnetite, garnet, hematite, rutile, etc., are explored for dating, sample selection and processing methodology must be refined. The use of High Resolution X-Ray Computed Tomography (CT) to screen grains or whole rock samples used for thermochronology is critical for opaque or complexly grown minerals. CT data, with associated software, provide a non-destructive method to visualize the internal structure of a sample in 3D, and precisely measure the volume. Internal visualization is particularly relevant to screen for inclusions or intergrowths that can affect parent and daughter nuclide distribution, and diffusion domain size. In order to accomplish a 3D petrographic study, CT scanning was coupled with thin section and Scanning Electron Microscope (SEM) analyses to fully characterize a suite of magnetites for potential (U-Th)/He dating. These analyses were able to determine 1) if matrix is attached to the grain after mineral separation, 2) the size and distribution of inclusions, 3) volume loss after physical air abrasion of samples, and 4) the textural relationship of the mineral in the whole rock. The distribution of minerals with low parent concentrations in relation to higher parent bearing-phases must be understood to account for He distribution, and to apply a proper correction for alpha injection/ejection. The results of this study demonstrate the importance of tailoring mineral separation and dissolution techniques to the mineral and rock type of interest.