THERMAL MODELING OF COOLING TIME OF SUBMARINE DACITE LAPILLI TUFFS: BROTHERS VOLCANO, KERMADEC ARC

The purpose of this study is to determine the eruption style of submarine arc volcanic eruptions using thermal modelling, specifically those that show evidence of pyroclastic eruptions at depths greater than 1500 meters below sea level. Even though these underwater eruptions are fundamental to the development and growth of island arc systems, very little is known about their eruptive histories and style of eruption. IODP Expedition 376 from the Brothers volcano, part of the Tonga-Kermadec arc, the largest island arc complex in the world, provides a great opportunity to study these eruptive systems with recovery through multiple eruptions down to approximately 450 meters below the seafloor. Two eruptive sites were the focus of this project: Site U1528 located on a resurgent cone and Sites U1527 and U1530 located on the rim of the caldera. Samples were analyzed for clast radius, density, thermal conductivity, and specific heat. This data was then used in a heat transfer equation of a sphere to calculate the time for a clast to cool from its initial eruption temperature to stable temperature.

Using three initial eruption temperatures of 800, 900, and 1000°C, the model shows the amount of time needed for a clast with a radius of 1 cm to cool. The model shows that with an initial eruption temperature of 800°C the clast cools below the Curie temperature for magnetite at a time of 200 seconds. The model solved for eruption temperatures of 900 and 1000 °C shows that the Curie temperature is achieved at intervals 300 and 400 second, respectfully. Previous paleomagnetism work on these locations show that the lapilli tuffs have a single magnetization, and they all have the same magnetic vector inclination. To achieve the observed paleomagnetic record and thermal modeling, clasts would be required to have erupted, settled on the sea floor in approximately 3 minutes. Settling time increases by one second for every increase in degree of initial eruption temperature. This suggests that submarine volcanoes can form lapilli tuffs in a single, pyroclastic eruption and does not require secondary brecciation processes.