Part 3 - Chemical trendsNow you will assess the variation in whole-rock chemical composition among the Quaternary volcanoes of the Central American arc. The data from the CAGeochem.xls file are not perfectly unbiased (i.e., certain rock compositions are oversampled relative to others), but it will do nicely for our purposes.
- Make a series of x-y plots with Distance along the x-axis and various chemical parameters on the y-axis. Be sure to include SiO2, K2O, and a few trace elements on the y-axis. What is the range of silica contents for the volcanic rocks along the arc? Is this range constant with location along the arc, or does it vary? What are a few of the variables that could be responsible for these changes?
- Some magmas in subduction zones carry geochemical signatures of components derived from the subducting slab, for example, subducted sediments or altered mid-ocean ridge basalt. A particularly sensitive signal for these sources is Ba/La. Ba is an incompatible element enriched in sediments and fluids, and La is a rare-earth element depleted in sediments and fluids. Make a plot of Distance (x-axis) vs. Ba/La (y-axis). A typical mantle values is 15. Do portions of the Central American arc exceed this value and suggest the presence of a subduction component in the sources of these magmas?