Part 2 - Investigate the 1959 Eruption

What was erupted?

In this section, you will answer a series of questions about the petrology of the basalts which were sampled during the 1959 eruption of Kilauea Iki. In order to answer the questions, you will use the data in the Surface samples worksheet to make some geochemical plots. Hints and answers are provided after each question.

---

1. Was the lava erupted during the 1959 eruption of Kilauea Iki compositionally homogeneous or heterogeneous?
2. How did the composition of the erupted lavas change during the course of the eruption? Was this change a continuous one, or does the change appear to be more complex? Did the various element oxides steadily increase or decrease during the eruption?
3. Following up on the previous question, what does this tell you about the nature of the source of the Kilauea Iki magmas?

Show hint

Hide

First, think about what you might expect if the eruption originated from a single magma chamber characterized by a simple gradient in composition, perhaps a vertical one produced by crystal fractionation.

4. The next few questions concern the possible origin(s) of the compositional variation in the erupted lavas (i.e., the processes operating in the magma chamber(s) which contributed magma to the eruption). Important pieces of information to consider at this point are the texture and mineralogy of the erupted basalts. Most of these samples are porphyritic with olivine phenocrysts in a finer-grained groundmass of clinopyroxene and plagioclase, but also containing minor amounts of Fe-Ti oxides, apatite, and orthopyroxene, and glass.
   
   First, make a plot of SiO2 (x-axis) vs. MgO (y-axis) of just the whole-rock lava compositions. Describe the shape of the geochemical trend defined by these samples.
   
5. Suggest a few petrologic hypotheses that would account for this trend shape.
6. Now, add the glasses to your graph. Where do the glasses fall relative to the trend observed for the lavas and why?
7. Evaluate the possibility that crystal fractionation of a Mg-bearing mineral is controlling the SiO₂-MgO trend displayed by the whole-rock analyses.

Show hint

Hide

Add the analyzed Mg-bearing minerals to your plot.

Show hint

Hide

Does the linear trend defined by the erupted basalts point towards either of the Mg-bearing minerals?

8. Why do the olivine analyses form a trend of their own on the graph?
9. Does fractionation of pure forsterite, pure fayalite, or some intermediate olivine composition explain the compositional variation among the erupted lavas?
10. What would the variation trend for the lavas look like if olivine fractionation had occurred over an extended range of temperature? Ignore the effects of other lower temperature phases (like clinopyroxene and plagioclase) for now.