Part 7 - Analyze and Interpret Your Results

Questions to answer

You should now have a number of variation diagrams plotted. Its time to try to make some sense out of all of this! The following are just a few question to get you thinking about what these diagrams may be telling you about the origins of the Yellowstone and Crater Lake volcanic rocks.

  1. Which of the two suites shows the most continuous range in composition?
  2. In which location would you expect to find andesites?
  3. In their study of the material from the 6845 BP climactic eruption of Mount Mazama, Bacon and Druitt (1988) noted that there is a compositional gap between 61-70 wt.% SiO2 in the whole rock data. This gap is not apparent from your larger dataset which includes rocks of other ages (particularly for the SiO2 vs. MgO plot which has more datapoints), but it does seem like there are noticeably fewer samples in this compositional region on your plots. However, Bacon and Druitt note that there is no gap in the 6845 BP dataset if volcanic glasses are plotted instead of the whole-rock data. How does this fact serve to explain the origin of the compositional gap observed by Bacon and Druitt? Hint: the rocks across this compositional range are porphyritic.
  4. What is the term that describes the range of compositons displayed by the Yellowstone Plateau Volcanic Field?
  5. Based on the spread of the Yellowstone data across SiO2 (or MgO), which is the better petrogenetic hypothesis for the origin of the Yellowstone rhyolites (note: neither one may actually be true): (1) fractional crystallization of basaltic liquids compositionally similar to the basalts in the Yellowstone dataset, or (2) partial melting of mafic rocks (gabbros) similar to the basalts in the Yellowstone data?
  6. What happens to the scatter of the data as a function of SiO2 for both the Yellowstone and Crater Lake datasets? Do you think this has petrologic significance? If so, what?
  7. What's going on with the wacko high-MgO samples from Crater Lake?
  8. Look for clues in the 'Sample Comment' colum in the downloaded spreadsheet.
  9. Let's focus on the incompatible elements K, Rb, and Ba right now. What minerals commonly found in igneous rocks usually host these elements?
  10. Describe the incompatible element (K, Rb, and Ba) trends for the Yellowstone rhyolites when plotted against SiO2 or MgO? What process(es) could be responsible for this interesting trend?
  11. Think about what would happen if the concentrations of these elements were solely controlled by magmatic processes involving minerals containing SiO2 and/or MgO.
    What about non-magmatic processes? Could these have affected the incompatible element concentrations?