Evolution of volatile species in the earth’s mantle: A view from xenology

International audienceTo understand the scales and chronology of processes governing the evolution of terrestrial gas species, the constraints from 244Pu–238U–129I–Xe systematics are crucial and should be included in any model related to gas loss/gain by the Earth and to gas redistribution among terrestrial reservoirs.Reliable constraints can be derived from meteoritic and terrestrial abundances of the highly refractory lithophile incompatible parent isotopes 244Pu (half life τ244 = 80.0 Myr) and 238U (τ238 = 4468 Myr). Both isotopes produce heavy Xe isotopes by fission; different relative yields for the Xe isotopes allow contributions of Xe(Pu) and Xe(U) to be distinguished. It is also useful to consider the 129I–129Xe(I) systematics (τ129 = 15.6 Myr) even though iodine is a highly volatile element and its terrestrial abundance is less well known. The parent isotopes, for which the initial (at the time of formation of the solar system, 4.567 Gyr ago) abundances are known from investigations of meteorites and ancient terrestrial zircons, yield the closed-system (subscript CLOS) present-day ratios of [136Xe(Pu)/136Xe(U)]CLOS = 28 and [129Xe(I)/136Xe(Pu)]CLOS = 110, much exceeding values observed in the depleted heterogeneous mantle reservoir (DMR): [136Xe(Pu)/136Xe(U)]DMR ⩽ 3 and [129Xe (I)/136Xe(Pu)]DMR ⩽ 60 (Pepin and Porcelli, 2006). Also, the present-day amounts of 129Xe(I) in the mantle (∼0.01 Tmol) and in the atmosphere (0.278 Tmol) are well below the total value produced by decay of 129I (129Xe (I) = 35 Tmol). These relationships between the closed system and the observed values show loss of early-produced Xe isotopes occurred not only from the DMR, but also from the Earth–atmosphere system as a whole.Using abundances of the parent and daughter isotopes within the framework of a simple one-mantle-reservoir degassing model we conclude: (1) the present day mantle is a severely degassed reservoir, so that only