Evolution of helium and argon isotopes in a convecting mantle

Outgassing of helium and argon and the isotope ratio distribution of helium isotopes in basalts are some of the most important geochemical constraints on mantle structure and evolution, but their interpretation in terms of mantle processes is ambiguous and controversial. Here, the evolution of these isotopes and their radiogenic parent isotopes of U, Th and K are studied using a numerical model of mantle convection that combines a treatment of major and trace element melting-induced differentiation and evolution with a self-consistent mantle convection-plate tectonics treatment. Both olivine and pyroxene-garnet system phase transformations are included, with density profiles of the different components following established data up to 720 km depth, but varied in the deeper mantle to reflect present uncertainties. Cases are presented that focus on the influence of two uncertain physical parameters: the density of subducted eclogite in the deep mantle, and the partition coefficient for helium. Results indicate that the system self-consistently evolves regions with the observed range of 3He/4He, but the exact distribution depends strongly on physical parameters. Furthermore, the distribution depends on sampling method, with the distribution in erupted material often being different from mantle-averaged distributions. Some parameter combinations simultaneously lead to MORB-like distributions of 3He/4He ratios in erupted material, and ∼50% outgassing of radiogenic 40Ar consistent with geochemical constraints. MORB-like 3He/4He histograms are produced in erupted material either when the shallow mantle has a high proportion of residue that evolves MORB-like 3He/4He due to the high incompatibility of He, or when sufficient recycled crust mixes back into the shallow mantle to suitably reduce its 3He/4He. Outgassing is also studied for Venus and Mars-like models, both of which are found to outgas substantially despite their lack of plate tectonics. The 25% outgassing of Venus proposed by [W.M. Kuala. Icarus, 139(1) (1999) 32] is plausible if Venus’ mantle viscosity is high or melt does not efficiently degass. It is estimated that Mars has lost ∼90% of its outgassed Ar to space, consistent with other estimates. © 2004 Elsevier B.V. All rights reserved.