Part 5 - Decide Which Variation Diagrams to Plot
BackgroundObviously, variation diagrams are alot more useful than looking at tables of whole-rock data from a suite of samples. At the same time, the proper use of variation diagrams is definitely an acquired skill. To the practiced eye, they not only display the range of petrologic variation, but they may also convey important clues about geochemical and petrogenetic processes. At the same time, variation diagrams have important limitations. The mere presence of a well-correlated trend on a variation diagram is usually not enough to uniquely identify a particular geochemical process. Stated another way, similar trends may result from more than one process. In the absence of unambiguous interpretations from major-element variation diagrams, additional data (e.g., trace-elements, rare-earth elements, and isotopes) may help discriminate between processes.
For a good discussion of variation diagrams, please consider reading p. 66-84 in Rollinson (1993) or p. 136-140 in Winter (2001) . For a web-based resource, consider downloading the Chapter 8 PowerPoint lecture in John Winter's Igneous and Metamorphic Petrology Class Materials (more info) .
Getting startedTo some extent, the choice of which variation diagrams to plot depends on what specific petrologic questions (hypotheses) you are asking of the data. On the other hand, you will usually need to see some data plotted up before you can start formulating those petrologic questions and hypotheses in the first place. So don't worry that you don't yet know what the diagrams will be used for. Plot a bunch of them up that share the same variable on the x-axis as a place to start. You can always plot additional or more useful diagrams later.
What goes on the x-axis?An important aspect of igneous suites (magmas presumably related in origin) is their range of chemical variation. This fundamental observation places limitations on the petrologic process(es) which could explain the origin of the suite. For the purpose of this exercise, we are mainly interested in comparing the petrologic ranges of the Yellowstone and Crater Lake datasets. To do this, we will need some indicator of petrologic variability to plot on the x-axis. Normally, the oxide that displays the greatest absolute range is chosen, and for most rocks, that oxide is SiO2. However, you should know that there are some graphical problems that result from choosing an oxide as abundant as SiO2 usually is in most rocks [Rollinson, 1993] . An alternative oxide to consider plotting on the x-axis is MgO.
A list of suggested diagrams to plotFor starters, I suggest making the following major-element variation diagrams for each caldera complex.
- SiO2 and/or MgO vs. TiO2
- SiO2 and/or MgO vs. Al2O3
- SiO2 and/or MgO vs. FeOT
- SiO2 vs. MgO
- SiO2 and/or MgO vs. CaO
- SiO2 and/or MgO vs. Na2O
- SiO2 and/or MgO vs. K2O
- SiO2 and/or MgO vs. P2O5
- SiO2 and/or MgO vs. Rb
- SiO2 and/or MgO vs. Sr
- SiO2 and/or MgO vs. Ba