EPISODIC BREACHING OF AN ACTIVE HYDROTHERMAL SYSTEM AND ASSOCIATED THERMAL PERTURBATIONS INDUCED BY GROUNDWATER FLOW: KILAUEA VOLCANO, HAWAII

It has been hypothesized that episodic breaching of deep hydrothermal systems within volcanoes leads to the ascent of large volumes of superheated fluid along faults within the edifice (Fournier, 1999). Such plumes could have a major impact on the thermal regime in the vicinity of the summit. Temperature profiles in wells in the vicinity of some volcanoes show relatively cool isothermal zones overlying zones of steep positive and negative thermal gradients. It is commonly assumed that the high-temperature sections are a consequence of ongoing lateral fluid flow through a thin aquifer, or (less commonly) free convection in a confined domain. However, in some volcanoes, the high-temperature zones are associated with extensive mineral alteration, implying low permeability. Here, we propose an alternative model to interpret such a temperature profile in a deep well on the summit of Kilauea Volcano, Hawaii. We suggest that breaching of the hydrothermal system by a major tectonic event triggered the ascent of a superheated fluid plume towards the surface. Because of the large horizontal permeability of unaltered Hawaiian basalt flows, part of the ascending fluid plume was channeled laterally at shallow depths, where rocks were as yet unaltered and relatively permeable. This lateral flow caused thermoelastic expansion, precipitation of secondary minerals, and alteration of the host basalt, reducing permeability and causing a relatively rapid transition from advection-dominated to conduction-dominated heat transport. The results of our numerical simulations suggest that a hydrothermal plume event may have occurred in Kilauea during the 18th or 19th centuries, and also demonstrate that thermal perturbations can be observed in a volcanic edifice long after flow has essentially ceased, consistent with evidence from fluid inclusions in secondary minerals.