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.
- 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.%).
- 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.
- Compare the MgO-time sequences of the erupted lavas with the MgO-depth stratification in the lava lake.
- This hypothesis does not explain the data. The trend in the erupted lavas is one of two stages of high MgO (olivine-rich) lavas preceded and separated by low MgO (olivine-poor) lavas. Except for the lower part of the lake section, the general trend in the lake from bottom to top is one of decreasing MgO followed by increasing MgO in the near-surface layers. Researchers believe that the original compositional heterogeneity in the erupted lavas was mostly eliminated (by mixing) during the course of the eruption itself (Wright, 1973; Helz, 1987b).
- 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?
- As in the erupted samples, most of the whole-rock data form a straight line pointing towards forsteritic olivine. Olivine fractionation (perhaps by crystal settling) is probably an important process in generating the fractionated whole-rock compositions. (For reasons beyond the scope of this exercise, however, it is not the only process to have occurred in the lake).
- Does olivine fractionation alone explain the glass compositions?
- 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)?
- Settling of dense olivine crystals from the top part of the lake to the bottom part of the lake.
- How might the enrichment of MgO (and hence olivine) in the upper 15 m of the lake be explained?
- Because of its proximity to the atmosphere, this is the part of the lake that cooled the most rapidly. We might then think that perhaps the rock was not molten for long enough to allow loss of olivine by settling. However, the fact that this rock contains MgO contents in excess of any erupted lava indicates that olivine must have actually accumulated this this part of the lake by some mechanism and then kept from sinking into deeper parts of the lake (perhaps because of the rapid cooling near the surface).
- 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?
- Considering their high SiO2 contents and low contents of MgO and FeO, theses liquids would form rhyolites. While rocks with rhyolitic bulk compositions were not formed at Kilauea Iki, other locations of voluminous basaltic volcanism, such as Iceland, do produce such fractionated compositions.
- 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?
- This is the classic tholeiitic trend. Notice the characteristic Fe-enrichment followed by Fe-depletion.