EXPLORATION FOR BLIND GEOTHERMAL RESOURCES IN THE STATE OF HAWAIʻI UTILIZING DISSOLVED NOBLE GASSES IN WELL WATERS

Abstract

This study is an extension of the Hawaii Play Fairway Analysis (PFA), a statewide geothermal exploration project funded by the United States Department of Energy. Based on results from prior phases of the PFA, this project targeted 66 wells on the islands of Hawai‘i, Maui, Lānaʻi, Oʻahu, and Kauaʻi for sampling of dissolved noble gases, trace metals, common ions, and the stable isotopes 2H and 18O. Ultimately, 23 of the 66 well targets were sampled. Noble gas data from this study is supplemented with data shared by the United States Geologic Survey for the summit of Kīlauea, and by the geothermal energy company Ormat Technologies Inc. for their geothermal power plant Puna Geothermal Venture on the Lower East Rift of Kīlauea, and for their exploration of Kona and Hualālai on Hawaiʻi, as well as the Southwest Rift of Haleakalā on Maui. The noble gas helium is used as an indicator of geothermal heat when excess 3He and/or 4He is present when compared to the atmospheric ratio of those isotopes (R/Ra). R/Ra is minimally affected by dilution and transport, allowing even those wells not perfectly situated over a geothermal system to indicate a geothermal anomaly. R/Ra anomalies are present on every island in this study. There is a strong correlation between R/Ra anomalies and proximity to rift zones and calderas. Across the islands R/Ra ranged from 15-16 on Kīlauea’s lower east rift zone, which is a mantle plume value, to 0.37 on Lāna‘i, which is a crustal value. The majority of anomalous well samples had R/Ra values consistent with an upper mantle source. Mixing between upper mantle and crustal helium is evident on all islands. Geographically, R/Ra decreases from the high at Kīlauea to upper mantle values at Mauna Loa, and remains at upper mantle values for all wells across the other volcanoes, with the exception of two of four sampled wells on Lānaʻi, which have radiogenic, crustal R/Ra values (<1). The trend of steeply decreasing R/Ra spatially, from plume-like to upper-mantle values, and then relatively constant values to Kaua‘i is consistent with fluid transport in the upper mantle in the direction of plate motion, and migration of noble gases into that fluid reservoir. Due to the association of helium with heat, it is likely that geothermal resources of undetermined potential are present on most Hawaiian Islands.