A geochemical model of the Kilauea East Rift Zone

Abstract

The east rift zone of Kilauea Volcano is comprised of a northeast- to southwest-trending complex of dikes and fractures extending more than 100 kilometers from the summit caldera to the ocean floor. Geologic, petrologic, and geophysical data indicate that substantial volumes of molten magma are intruded into and stored within the east rift dike complex and that parts of the rift have temperatures exceeding the Curie point of basalt. The shallow ground-water hydrology and chemistry on the lower rift are strongly affected by natural thermal discharge from the rift and indicate a continuous heat at a rate estimated at 291 megawatts of thermal energy. Several deep geothermal wells drilled into the lower rift have confirmed the presence of high temperatures and of an active hydrothemeral system associated with the rift. The maximum temperatures encountered in the deep wells approach the critical point of water (374 C) but show a sharp decline on the southern boundary of the rift. Petrologic studies of drill cuttings from the deep geothermal wells have shown that intermittent, intense geothermal alteration has occured to depths of at least 2.5 km. The alteration phases present indicate that metmorphism reaches the greenschist facies and have strong similarities to suites found in midocean-ridge hydrothermal systems. Chemical data from the deep fluids suggest that the primary source of recharge to the reservoir is meteoric water in the interior of the rift but that saline water is present on the northern boundary. Production data from the deep wells also indicate that accessible parts of the hydrothermal system are capable of producing both dry steam and hot brine. The data that have been gathered to the present indicate that the hydrothermal system associated with the Kilauea east rift zone is evolving and has characteristics ranging from low-temperature benigh fluids to high-temperature, highly aggressive fluids that may provide both an economically viable geothermal resource and a natural laboratory for the study of transformatin and geochemical-cycling processes.