2-D numerical simulations of groundwater flow, heat transfer and 4He transport — implications for the He terrestrial budget and the mantle helium–heat imbalance

Because helium and heat production results from a common source, a continental 4He crustal flux of 4.65 * 10− 14 mol m− 2 s− 1 has been estimated based on heat flow considerations. In addition, because the observed mantle He / heat flux ratio at the proximity of mid-ocean ridges (6.6 * 10− 14 mol J− 1) is significantly lower than the radiogenic production ratio (1.5 * 10− 12 mol J− 1), the presence of a terrestrial helium–heat imbalance was suggested. The latter could be explained by the presence of a layered mantle in which removal of He is impeded from the lower mantle [R.K. OʹNions, E.R. Oxburgh, Heat and helium in the Earth, Nature 306 (1983) 429–431; E.R. Oxburgh, R.K. OʹNions, Helium loss, tectonics, and the terrestrial heat budget, Science 237 (1987) 1583–1588]. van Keken et al. [P.E. van Keken, C.J. Ballentine, D. Porcelli, A dynamical investigation of the heat and helium imbalance, Earth Planet, Sci. Lett. 188 (2001) 421–434] have recently claimed that the helium–heat imbalance remains a robust observation. Such conclusions, however, were reached under the assumption that a steady-state regime was in place for both tracers and that their transport properties are similar at least in the upper portion of the crust. Here, through 2-D simulations of groundwater flow, heat transfer and 4He transport carried out simultaneously in the Carrizo aquifer and surrounding formations in southwest Texas, we assess the legitimacy of earlier assumptions. Specifically, we show that the driving transport mechanisms for He and heat are of a fundamentally different nature for a high range of permeabilities (k ≤ 10− 16 m2) found in metamorphic and volcanic rocks at all depths in the crust. The assumption that transport properties for these two tracers are similar in the crust is thus unsound. We also show that total 4He / heat flux ratios lower than radiogenic production ratios do not reflect a He deficit in the crust or mantle original reservoir. Instead, they reflect the combined impact of air saturated water (ASW), advection, conduction, and diffusion when steady-state is reached for both tracers. We thus argue that the observed low mantle He / heat flux ratio in the oceans might be, at least partially, the result of processes occurring in the oceanic crust similar to those occurring in the continental crust, rather than deeper into the mantle. mulations also indicate that in order for both heat and He to be in steady-state in recently formed crust, the presence of an advective dominated regime is required (k ≥ 10− 16 m2). Under these conditions, only in total absence of contact with ASW (e.g., an atmospheric component provided by freshwater or seawater) is the total 4He / heat flux ratio expected to equal the radiogenic production ratio. Lower 4He / heat fluxes in an advective dominated regime require the incorporation of an ASW component. We argue that the observed low ocean mantle 4He / heat flux results, at least partially, from sea water incorporation within mid-ocean ridge basalts. Our simulations also suggest that 4He transport is in transient state in recently formed crust for permeabilities ≤ 10− 17 m2. Under these conditions, low to very low mantle He excesses and thus total He / heat fluxes of up to several orders of magnitude lower than the radiogenic production ratios are expected.