Teaching with the EarthChem Geochemical Database
Integrating Research and Education > EarthChem > Kilauea Iki Lava Lake > Step-By-Step Instructions > Part 2 - Investigate the 1959 Eruption

Part 2 - Investigate the 1959 Eruption


Photo: U.S. Geological Survey

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?
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    You don't even need to make any plots to answer this one. A simple inspection of the whole-rock data in the Surface samples worksheet shows that a number of the major-element oxides vary by significant amounts in the erupted lavas: MgO is particularly variable from 6.74 to 19.52 wt.%.
  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?
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    A plot of MgO vs. time shows that the lava compositions did not 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?
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    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.
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    The lack of a simple change in composition during the eruption is evidence against either (1) a simple (vertical) compositional gradient in the magma chamber at depth, and/or (2) a single magma chamber. In fact, researchers think two distinct chambers may have been involved in the eruption (Wright, 1973).

  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.

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    The samples define a linear trend.
  5. Suggest a few petrologic hypotheses that would account for this trend shape.
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    There are several possibilities: (1) mechanical mixing of two end-member magmas, (2) crystal fractionation dominated by the removal of a single solid phase, and (3) crystal fractionation involving more than one phase, but only if the proportions of the various phases remained constant during fractionation.
  6. Now, add the glasses to your graph. Where do the glasses fall relative to the trend observed for the lavas and why?
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    The glasses are restricted to the higher SiO2, lower MgO end of the trend. This is because the rocks are porphyritic and contain mafic minerals rich in MgO and poor in SiO2. The glasses are quenched volumes of melt which formed after the olivine phenocrysts and microlites precipitated from the mamga. Most of the compatible elements like MgO are depleted in these glasses because they are concentrated in mafic minerals.
  7. Evaluate the possibility that crystal fractionation of a Mg-bearing mineral is controlling the SiO2-MgO trend displayed by the whole-rock analyses.
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    Add the analyzed Mg-bearing minerals to your plot.
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    Does the linear trend defined by the erupted basalts point towards either of the Mg-bearing minerals?
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    The linear trend of the erupted basalts points toward olivine
  8. Why do the olivine analyses form a trend of their own on the graph?
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    The olivines form a trend because of the mineral's solid solution between Mg (forsterite) and Fe (fayalite) endmembers. Fo is the stable form of olivine at high temperatures, while Fa is the stable form at low temperatures.
  9. Does fractionation of pure forsterite, pure fayalite, or some intermediate olivine composition explain the compositional variation among the erupted lavas?
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    Since the trend for the lavas is linear and points towards the middle of the trend, this indicates that an intermediate olivine containing both forsterite and fayalite component was involved in the fractionation process. This olivine composition is still fairly rich in forsterite component (~ Fo85).
  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.
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    The variation trend of the lavas would be curved concave upward. Early Fo-rich olivine fractionated at high temperature would give way to later Fa-rich olivine fractionated at lower temperature. By the lever rule, the melts would move away from a progressively lower Mg, higher Fe fractionating solid.

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