Part 3. Investigate the Compositional Stratification Within the Lava Lake

Photo: Pat Holleran

What's down there?

In this section, you will answer a series of questions related to the drill core data from the Kilauea Iki lava lake, found on the Composite section worksheet. Most of the questions will require making some simple geochemical plots. Hints and answers are provided after each question.

  1. Earlier, you learned that MgO was an important oxide to interpret the origin of the chemical variation in the erupted lavas. Make a plot of MgO (x-axis) vs. drill core depth (y-axis, values in reversed order). Describe the changes in MgO with depth in the Kilauea Iki lava lake, particularly with respect to the "average" MgO content of the erupted lavas (~15.43 wt.%).
  2. You have also determined that the lavas erupted at Kilauea Iki were compositionally heterogeneous. Evaluate the hypothesis that the chemical variation in the interior of the lava lake is solely a result of the changing composition of the erupted lava. Remember that the lavas erupted early during the month-long eruption comprise the deepest layers within the lava lake.
  3. Some other mechanism besides variation in the initial lavas must be responsible for the trends in the drill cores. To investigate this, make a MgO vs. SiO2 variation diagram using the whole-rock, glass, and olivine analyses from the drill cores. Is there any evidence for olivine fractionation in the lava lake?
  4. Does olivine fractionation alone explain the glass compositions?
  5. Consider the shape of the MgO profile through the lake. If olivine fractionation was the only process operating in the lake (researchers have determined that other processes were also involved: Helz et al., 1989), how might you explain the depletion of MgO in the upper part of the section (from 15-35 m), and the underlying enrichment of MgO (from 35-75 m)?
  6. How might the enrichment of MgO (and hence olivine) in the upper 15 m of the lake be explained?
  7. If the most fractionated liquids (now glasses) from the Kilauea Iki lava lake were somehow able to migrate out of their crystal-liquid mushes, erupt, and then solidify into rock, what type of rock would be produced?
  8. The plot on the right is a ternary AFM diagram (A = alkalis, F = FeOT, M= MgO) of the whole rock (organge squares) and glass samples (blue squares) from the drill cores. What type of differentiation trend is this?