Is "Primordial" Helium Really Extraterrestrial?

Among the isotope systems exploited by mantle geochemists to deduce the chemical evolution of the Earth, those of the rare gases are unique because they record the migration of volatile species from the mantle to the atmosphere. The isotopic composition of helium in mantle-derived rocks is now reasonably well known, and it suggests that this migration has been both more heterogeneous and less extensive than commonly expected. The implications of the helium data are profound, but they seem to conflict with widely held beliefs derived from other geochemical tracers, including the isotopic characteristics of lithophile elements (that is, elements such as Sr, Nd, and Pb found predominantly in Earth's silicate rust). Although the geochemical processes affecting the rare gases and lithophile species must be intimately related, it has proven difficult to reconcile them in any obvious way. On page 170 of this issue, Anderson offers a controversial hypothesis that can simultaneously explain all of these disparate tracers (1). If he is correct, a dramatic reinterpretation of the terrestrial rare gas data and their significance to planetary evolution is required