ON TIME: ACTIVE LEARNING WITH FISSION TRACK ANALYSIS

Students are typically introduced to Geologic Time in physical geology classes, in which they learn that geologic time is immense and that geologists have ways to determine the relative and absolute ages of geologic processes and materials. Geologic time is central to geoscience teaching and research, but because of the complexity of using mass spectrometers, undergraduate students are seldom involved in actual measurement of ages of rocks or minerals or geologic processes.

Apatite fission track (AFT) analysis, however, is an accessible way to involve students in determining dates for geologic events. The exercise can be incorporated into a variety of physical or historical geology classes or can be run as a separate workshop on geologic time or tectonics. Depending on time and access to facilities, the exercise can be done on 'unknowns' - rocks for which the apatite fission track age is not known - or on pre-separated apatite standards of known age and U-content. In the case of working with unknowns, mineral separation techniques for isolating apatite are useful for teaching about the physical properties of minerals (for example, density, and magnetic susceptibility). We will describe and illustrate 3 possible AFT modules: (1) Counting only (using pre-separated apatite of known uranium content); (2) Intermediate module (eliminates irradiation step but still involves some sample preparation); and (3) Rock to grain mount (mineral separation, determination of U concentration, counting).

An added advantage of teaching students about apatite fission track analysis is that it provides an excellent illustration of the concept of temperature-time paths for rocks. Metamorphic rocks and some plutonic igneous rocks don't have a single age. The relationships of temperature, time, and processes can be well illustrated by determining the apatite fission track age of a metamorphic or slowly cooled plutonic igneous rock of known crystallization age.