A dynamical investigation of the heat and helium imbalance

The terrestrial heat-helium imbalance [O'Nions and Oxburgh, Nature 306 (198 3) 429-431] is based on the observation that significantly less He-4 is rel eased from the Earth's mantle than is predicted from the radiogenic element budget and observed heat now. We review recent observations and models of Earth's radioelement distribution and He-4 flux and demonstrate that this i mbalance remains a robust observation. We explore the hypothesis that the i mbalance can be accounted for by different timescales of heat and helium ex traction from the mantle system. This is tested using dynamical models of m antle convection that incorporate thermal evolution, helium ingrowth and de gassing. The temporal decoupling of heat and helium loss provides large exc ursions from the mantle heat and helium production ratio and can indeed dro p to values as low as those observed. Nevertheless, the duration of these p eriods is very limited within the 4 Byr model period and the probability th at the present-day situation is caused by such an excursion must be conside red to be very small. While the average ratio of heat and helium released f rom the whole mantle convection models is smaller than the production ratio , a significant imbalance remains. An additional mechanism is required to f urther separate heat from helium. (C) 2001 Elsevier Science B.V. All rights reserved.