![]() An introduction to statistical inference from small samples can also be added to this exercise. The (pre-counted) unknown quantity of beads represents the isotopic composition of zircon from four samples-the Deccan Trap basalts, the Chicxulub impact melt, and ash layers above and below the K-T boundary -and the students' measurements are used in the final part of the module. Here the students determine the number of beads in a large bucket without counting them all by adding a precisely known number of "tracer" beads and averaging ratios from several small samples of the mixture. Next, the technique's accuracy despite loss of parent and daughter atoms during analysis, as well as the use of isotopic ratios rather than absolute abundances, is explained with an activity on isotope dilution. Manipulating groups of double-sided chips labeled with U and Pb isotopes reinforces the concept that an age determination depends on the Pb/U ratio, not the absolute number of atoms present. In this module, the students start by exploring the concepts of half-life and exponential decay and graphically solving the isotopic decay equation. The curriculum teaches the science behind uranium-lead dating using tables, graphs, and a geochronology kit. In the second part, we use a curriculum developed for and available on the EARTHTIME website (). Students also discuss geologic features relevant to the K-T boundary problem and get the chance to examine basalts, impact melts and meteorites. We cover all the steps from sampling an outcrop to determining a final age. Students use different techniques, such as magnetic separation, density separation using non-toxic heavy liquids, and mineral identification with a microscope. ![]() Activities are divided into three parts: In the first part, the instructors lead hands-on activities demonstrating how rock samples are processed to isolate minerals by their physical properties. The theme of our 2.5-hour module is the timing of the K-T boundary and a discussion of how geochronology can be used to evaluate the two main hypotheses for the cause of the concurrent extinction-the Chicxlub impact and the massive eruption of the Deccan Traps. The workshop focuses on uranium-lead (U-Pb) dating of zircons and its application to solving a geological problem. The authors taught an educational module developed as part of the EARTHTIME (outreach initiative to 215 high school students from a Massachusetts (USA) High School as part of an "out-of-school" field trip.
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