A model to explain the various paradoxes associated with mantle noble gas geochemistry

As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer (≈1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess “primordial” (3)He is responsible for (3)He/(4)He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of (3)He and excessive depletion of (4)He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the (3)He/(4)He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO(2) and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High (3)He/(4)He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.