S. Bloomberg | Geology Reports

USGS2015

Heat flux from magmatic hydrothermal systems related to availability of fluid recharge

Magmatic hydrothermal systems are of increasing interest as a renewable energy source. Surface heat flux indicates system resource potential, and can be inferred from soil CO 2 flux measurements and fumarole gas chemistry. Here we compile and reanalyze results from previous CO 2 flux surveys worldwide to compare heat flux from a variety of magma-hydrothermal areas. We infer that availability of water to recharge magmatic hydrothermal systems is correlated with heat flux. Recharge availability is in turn governed by permeability, structure, lithology, rainfall, topography, and perhaps unsurprisingly, proximity to a large supply of water such as the ocean. The relationship between recharge and heat flux interpreted by this study is consistent with recent numerical modeling that relates hydrothermal system heat output to rainfall catchment area. This result highlights the importance of recharge as a consideration when evaluating hydrothermal systems for electricity generation, and the utility of CO 2 flux as a resource evaluation tool.

Journal of Volcanology and Geothermal Research↗

USGS2014

Soil CO2 emissions as a proxy for heat and mass flow assessment, Taupō Volcanic Zone, New Zealand

The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high-resolution measurement of carbon dioxide flux (φCO 2 ) and heat flow at the surface to characterize the mass (CO 2 and steam) and heat released to the atmosphere from two magma-hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3000 direct measurements of φCO 2 and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO 2 emission rates (ΣCO 2 ) of 441 ± 84 t d −1 and 124 ± 18 t d −1 were calculated for Rotokawa (2.9 km 2 ) and for the crater floor at White Island (0.3 km 2 ), respectively. The total CO 2 emissions differ from previously published values by +386 t d −1 at Rotokawa and +25 t d −1 at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO 2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal-sourced CO 2 emission (constrained using stable isotopes) with reservoir H 2 O:CO 2 mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MW t ) is 10 times greater than the White Island crater floor (22.5 MW t ).

Taupō Volcanic Zone↗