Modeling the sharp compositional interface in the Pu`u `Ō`ō magma reservoir, Kīlauea volcano, Hawai`i

Lavas from the early episodes of the Pu`u `Ō`ō eruption (1983–1985) of Kīlauea Volcano on the island of Hawai`i display rapid compositional variation over short periods for some episodes, especially the well‐sampled episode 30 with ∼2 wt% MgO variation in <4 hours. Little chemical variation is observed within the episode 30 lavas before or after this abrupt change, suggesting a sharp compositional interface within the Pu`u `Ō`ō dike‐like shallow reservoir. Cooling‐induced crystal fractionation in this reservoir is thought to be the main control on intraepisode compositional variation. Potential explanations for a sharp interface, such as changing reservoir width and wall rock thermal properties, are evaluated using a simple thermal model of a dike‐like body surrounded by wall rock with spatially variable thermal conductivity. The model that best reproduces the compositional data involves a change in wall rock thermal conductivity from 2.7 to 9 W m −1 C −1 , which is consistent with deep drill hole data in the east rift zone. The change in thermal conductivity may indicate that fluid flow in the east rift zone is restricted to shallow depths possibly by increasing numbers of dikes acting as aquicludes and/or decreasing pore space due to formation of secondary minerals. Results suggest that wall rock thermal gradients can strongly influence magma chemistry in shallow reservoirs.