SUPERHEATING

Heating a rock above its liquidus temperature has significant effects on the morphology and composition of crystals formed upon cooling.

  • Binary image of olivine crystals grown at low superheating, showing low surface area per unit volume (SvP)
  • Binary image of olivine crystals grown at high superheating, showing high surface area per unit volume (SvP)

Working with Julia Hammer and Tanis Leonhardi (former undergrad in the lab, now PhD candidate at Berkeley), we explored the effects of superheating on a silica-undersaturated rock with olivine on the liquidus. Based on previous work (e.g., Colin Donaldson’s papers) we expected to see morphological differences with different degrees and durations of superheating, but quantification of these differences, via the surface area per unit volume of the olivine crystals, had not yet been applied to other superheating experiments. We searched as well for evidence of compositional effects of superheating, but for now it appears that superheating does not yield a distinct compositional signature in subsequently-grown olivine.

  • superheating
    Relationships between crystal morphology (SvP, x-axis), and various measures of superheating. These plots show that the TtL parameter (a), which is the degree of superheating integrated over time, is a far superior measure of the overall magnitude of superheating than either temperature (b) or duration (c), alone.

Moral of the story? Varying superheating treatments cause morphological differences in olivine crystals on a scale similar to that induced by varying cooling rates. Therefore, it is essential for future lab work to address superheating explicitly as an experimental variable, rather than buried as a step in the sample preparation process. For natural samples, this means that if cooling rates can be determined by other means, crystal texture could theoretically provide a “superheat-o-meter” for the magma!

Publication: First, E., Leonhardi, T., Hammer, J. (2020) Effects of superheating magnitude on olivine growth. Contributions to Mineralogy and Petrology, 175: 13.