FROM THE INDOOR LAB TO THE OUTDOOR LAB: USING XRF DATA IN AID OF INTERPRETING FIELD RELATIONSHIPS

Many geology departments at N. American universities contain a plethora of sophisticated instruments for the analysis of rocks, minerals, waters, and other kinds of earth materials. The questions arise as to how to train students effectively in the use of such equipment, and how to demonstrate that their analytical results can be used to address real geological problems. One approach developed in the XRF lab at WashU involves a semester-project in a graduate course taught by RFD, ?Chemical Petrology.? St. Louis is situated close to the St. Francois Mountains, the structural culmination of the Ozark Dome, where the only significant exposures of Precambrian crystalline rocks in the mid-continent region are found. The bedrock consists almost entirely of ~1.4 Ga granites and rhyolites, exposed in a series of nested caldera complexes. Students are each assigned 4-6 closely related samples to investigate, collected previously by the instructor. They are given thin sections of each sample, accompanied by a sawn rock slab. The students learn how to process the slabs for analysis (i.e., by using crushers and a shatterbox to make powders). They are then shown by the lab manager (RAC) how to prepare fused-glass discs and pressed-powder pellets for major- and trace-element XRF analyses, respectively. (The students also prepare a pellet and disc from an in-house working standard, which allows for later comparison among results obtained by each person.) Once the analyses are completed, the students are invited to characterize and interpret their data using all of the theoretical and practical approaches developed in the lecture part of the course. Each student then makes a short presentation on their sample suite to the class, and the class then considers the results in their entirety. The most interesting part of this exercise comes later when the class visits the localities from which the samples were collected. Each student leads a discussion on the outcrop, wherein they consider the petrogenesis of ?their rocks? in light of petrographic observations and evaluation of their chemical analyses. The net result is that everyone, including the instructor, is humbled by the rocks! Nevertheless, this exercise enhances the learning experience for the students because it establishes a close connection between real data and real rocks.