The thermal history of a spatter-fed lava flow: the 8-ka pantellerite flow of Mayor Island, New Zealand

The thermal history of the 8-ka spatter-fed lava flow of Mayor Island, New Zealand, has been investigated via relaxation geospeedometry. Cooling rates extant during the vitrification of the clastogenic pantellerite flow have been quantified. Cooling rates modelled for two obsidian layers at the top and the base of the flow, vary over four orders of magnitude. The flow base defined by a lower obsidian layer shows a gradual increase in cooling rates from 0.00072 K/min adjacent to the crystalline core of the flow to 0.017 K/min at the contact to a basal crumble breccia. Cooling rates of flow ridges formed by an upper obsidian layer vary between 0.001 and 6 K/min. Based on field observation and thermal modelling, cooling rates of Kelvins to tens of Kelvins per day for both upper and lower obsidians are difficult to interpret as simple cooling processes. Instead, we propose a thermal history involving thermal annealing as an alternative explanation for these slow effective cooling, rates. Such thermal annealing appears to have facilitated welding and rheomorphism of the spatter deposit leading to the development of a coherent lava sheet. Based on the results obtained here, we present a quantitative scenario of the thermal history during the final stages of flow emplacement as a contribution to understanding the rheology of rheomorphic spatter deposits associated with explosive peralkaline rhyolitic volcanism and briefly address the hazard potential of such deposits.