Thermal history of Hawaiian pāhoehoe lava crusts at the glass transition: implications for flow rheology and emplacement

We have investigated the thermal history of glassy pāhoehoe crusts across their glass transition. Ten different samples obtained between 1993 and 2003 from the active flow field of the Puʹu ʹOʹo-Kupaianaha eruption on Hawaii (USA) have been analysed using relaxation geospeedometry. This method employs differential scanning calorimetry to quantify the enthalpic relaxation of the glass to monitor the natural time–temperature (t–T) path followed by the melt during cooling across its glass transition. Cooling rates across the glass transition interval (at 1000– 900 K) have been found to vary between 8 and 140 K/min. The associated glass transition temperatures are up to 400 K, lower than previously anticipated by others. Melt viscosities at the glass transition for these crusts range from 109.4 to 1010.7 Pa s. We have compared the t–T paths quantified via relaxation geospeedometry with those obtained from direct measurements on the active flow field. The calorimetrically determined cooling rates are consistent with either simple cooling from eruption temperatures to temperatures below the glass transition or more complex cooling paths, including periods of reheating and short-term annealing within the glass transition interval. By quantifying the relaxation times associated with these contrasting cooling histories, we show that secondary vesiculation of pāhoehoe flow crusts may be favoured by complex, nonlinear t–T paths within the glass transition. These constraints also allow us to evaluate the time scales associated with the crystallisation and inflation of flow lobes at the glass transition for different pāhoehoe lava flow types. Our results provide important quantifications of rheological parameters at the lower temperature range of viscoelastic deformation in basaltic lava flows. As such, the results may be helpful in refining models for the generation of continental flood basalt flows, as well as models of basaltic lava flow propagation for hazard assessment at active volcanic areas.