HR: 1340h
AN: H33H-1711    [Abstracts]
TI: Coupled Transport of Magma- and Mantle-Sourced Heat and Helium in Heterogeneous, Fractured Aquifers
AU: Andrews, J L
EM: andre345@umn.edu
AF: University of Minnesota Department of Geology & Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: * Saar, M O
EM: saar@umn.edu
AF: University of Minnesota Department of Geology & Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AB: Coupled transport of water and mantle-sourced heat and helium (He) is simulated in order to gain a better understanding of the patterns of temperature, He concentrations, and He isotope ratios (R=³He/⁴He) observed in groundwater systems where fault structures impact fluid transport, such as grabens, calderas, and volcanic regions. We consider the effects of implementing temperature- and mass-dependent He diffusion coefficients on He signals, as well as permeability, heterogeneity, buoyancy-driven recirculation, and radiogenic heat and ⁴He production in a variety of one-, two-, and three-dimensional systems with temperature gradients ranging from ~33-90~°C/km. The results of our investigation have applications to geothermal reservoir analyses and studies using heat and/or helium as natural tracers of groundwater flow. We find that even for permeabilities below 10^-15~m², inclusion of the temperature- and mass- dependence on He diffusion can have a large impact on patterns of He concentration and isotope ratios. Further, due to the large difference in the diffusion coefficients of heat and He (~3~orders of magnitude higher for heat) magma-sourced heat and helium signals can be spatially separated, or decoupled, for low-permeability systems in which He transport is dominated by advection, while heat transport is predominately conductive. Even for systems in which heat and He transport are both dominated by advection, temperature and He patterns may not be perfectly synchronized, again due to the wide difference between heat and He diffusion coefficients; while He patterns may be in-line with the dominant flow patterns, temperature profiles tend to be more diffused. Additionally, since He has a low diffusion coefficient, it is prone to entrapment by low-permeability layers, such as crystalline basements. This entrapment of He allows for high concentrations of He in low-permeability layers, such that these layers act as reservoirs of both mantle and crustal He. The depth to these low-permeability layers, therefore, can have a large impact on observed near-surface He signals. Because of the higher diffusion coefficient of ³He relative to ⁴He, ³He is able to escape more easily, creating lower R-values in these entrapment zones. Buoyancy-driven recirculation cells within a fracture system can lead to decoupling of heat and He isotope ratios, while leaving heat and He concentration relatively well-correlated. Radiogenic processes can obscure these nuances of magmatic heat and He transport by drastically altering the pattern and values of He signals throughout a groundwater system. Our results emphasize the importance of combining temperature, He concentration, and He isotope ratio data towards interpretation of groundwater flow patterns based on these natural tracers.
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1832 Groundwater transport
DE: 1847 Modeling
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Hydrology [H]
MN: 2007 Fall Meeting