Exploring for geothermal resource in a dormant volcanic system: The Haleakala Southwest Rift Zone, Maui, Hawai'i

Suites of new geophysical and geochemical surveys provide compelling evidence for geothermal resource at the Haleakala Southwest Rift Zone (HSWRZ) on Maui Island, Hawai'i. Ground-based gravity (~400 stations) coupled with heli-borne magnetics (~1500 line kilometers) define both deep and shallow fractures/faults while also delineating potentially widespread subsurface hydrothermal alteration on the lower flanks (below approximately 1800 feet a.s.l.). Multi-level, upward continuation calculations and 2-D gravity and magnetic modeling provide information on source depths, but lack of lithologic information leaves ambiguity in the estimates. Lithology and physical property data from future drilling will improve these interpretations. Additionally, several well-defined gravity lows (possibly vent zones) lie coincident with magnetic highs suggesting the presence of dike intrusions at depth; a potentially young source of heat for a modern geothermal system. Soil CO₂ fluxes were measured along transects across geophysically-defined faults and fractures as well as young cinder cones along the HSWRZ; a weak anomalous flux signal was observed at one young cinder cone location. Dissolved inorganic carbon concentrations and δ13C compositions and ³He/⁴He values measured in several shallow groundwater samples indicate addition of magmatic CO₂ and He to the groundwater system. The general lack of observed magmatic surface CO₂ signals on the HSWRZ is therefore likely due to a combination of groundwater 'scrubbing' of CO₂ and relatively high biogenic surface CO₂ fluxes that mask magmatic CO₂. Similar surveys at the Puna geothermal field on the Kilauea Lower East Rift Zone (KLERZ) also showed a lack of surface CO₂ flux signals attributed to a magmatic source, while aqueous geochemistry indicated contribution of magmatic CO₂ and He to shallow groundwaters at both Maui and Puna. As magma has been intercepted in geothermal drilling at the Puna field, the lack of measured surface CO₂ flux associated with upflow of magmatic fluids here is likely due to the aforementioned 'scrubbing' from extensive groundwater flow, as well as high background biogenic CO₂ flux. Deep, temperature gradient core holes have been sited based on these geophysical and geochemical datasets.