Spontaneous Reheating of Crystallizing Lava: An Experimental Study and Implications for Lava Fountains on Io

Abstract:

We have conducted experiments which show that recalescence, or spontaneous reheating of a cooling material due to rapid release of latent heat, can occur during disequilibrium crystallization of depolymerized Mg-rich melts. This can only happen at fast cooling rates, where the melt becomes undercooled by tens to hundreds of degrees before crystallization begins. Using visible and FLIR cameras, we have documented recalescence in pyroxene (Fe,Mg)SiO₃ and komatiite lavas, initially cooling at 25-50˚C/s. Local heating at the crystallization front exceeds 150˚C for the pyroxene, and 10˚C for komatiite, and lasts for 2-4 seconds as the crystallization front migrates across the sample (crucible ~25mm radius). The crystals are hotter than the melt from which they grew, and the crystallization front is the hottest part of the sample (Figure 1). We determined the latent heat release by differential scanning calorimetry, at 440J/g for pyroxene and 275 J/g for komatiite, with a power output of ~100 Wg^-1 or ~300 MWm^-3. Recalescence may be a widespread process in the solar system, on a range of scales, from the surfaces of ultramafic to mafic lava flows, and of vigorously convecting lava lakes, to lava fountains and chondrules. The most likely place for recalescence to be occurring at the present time is (ultra)mafic lava fountains on Io. If observed power outputs up to 25-30 GW from eruptions on Io were due entirely to recalescence of komatiite lava, this would suggest an effusion rate of ~75-100m³s^-1, similar to fissure 8 at Kilauea in 2018. Actual fluxes may be lower, because sensible heat also makes a large contribution, or higher, because only some droplets will reach the top of the fountain. We conclude that cooling histories of depolymerized silicate melts should not be assumed a priori to be monotonic, and that thermal imaging of lava fountains and flows needs to be conducted with high spatial resolution to assess true temperature fluctuations.