HR: 09:00h
AN: V11B-04    [Abstracts]
TI: Constraints on Mantle Convection and Atmospheric Escape in the Hadean
AU: * Yokochi, R
EM: yokochi@crpg.cnrs-nancy.fr
AF: Centre de Recherche Petrographiques et Geochimiques, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: * Yokochi, R
EM: yokochi@crpg.cnrs-nancy.fr
AF: Dept. of Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, Room 2456, Chicago, IL 60607-7059 United States
AU: Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: Centre de Recherche Petrographiques et Geochimiques, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: Ecole Nationale Superieure de Geologie, rue du Doyen Marcel Roubault, BP40, Vandoeuvre, 54501 France
AB: Mantle convection is intimately linked with terrestrial degassing through decompression melting, magma generation. Hence tracers of mantle degassing through time shed light on mantle dynamics from the Hadean to Present. Xenon isotopes provide an exceptional set of such tracers: ¹²⁹Xe was produced by the now extinct β-decay of ¹²⁹I with T_1/2= 15.7 Myr, ^131-136Xe were produced by the now extinct spontaneous fission of ²⁴⁴Pu with T_1/2= 82 Myr. ^131-136Xe are also produced by the extant spontaneous fission of ²³⁸U which decays with T_1/2= 4.45 Gyr. However, the respective proportions of ²⁴⁴Pu and ²³⁸U fissions to ^131-136Xe isotopes are poorly constrained because the fission of ²⁴⁴Pu and ²³⁸U produce the same xenon isotopes at only slightly different proportions. Instead of using the xenon isotopic composition, we determined the proportions of ²⁴⁴Pu and ²³⁸U fission derived ¹³⁶Xe according to the relative abundances of ⁴He-²¹ Ne-¹³⁶Xe, for a set of new noble gas data obtained for Kola plume (Russia) samples.
The ²⁴⁴Pu contribution of mantle plume-related component is estimated to be 33-60 % of total fission ¹³⁶Xe. This value is significantly lower than what is expected for a chondritic reservoir (96 %), and is equal to, or higher than, the previously proposed best estimate for the MORB mantle (32 ± 10 %). This result implies that whole mantle region was subject to degassing after the decay of ²⁴⁴Pu. As constrained by the half-life of ²⁴⁴Pu, such loss post-dated the major episodes of terrestrial accretion and and occurred mostly during the Hadean. Indeed, the mantle source has lost ≥ 99 % fission Xe produced by the decay of ²⁴⁴Pu while the loss of noble gas isotopes produced by long lived radioactivities (e.g., ⁴He, ⁴⁰Ar) is at least one order of magnitude lower, implying that the major loss of fissiogenic Xe isotopes from ²⁴⁴Pu took place before quantitative accumulation of noble gas isotopes produced by long-lived radioactivities. Both mantle plume and MORB sources were affected by such drastic fissiogenic Xe loss, suggesting whole mantle and very active convection early in the Earth's history. This view is consistent with a recent modelling of the helium isotopic variations in the mantle which concluded to whole mantle convection and degassing during the first half of the Earth's history (Class and Golstein, 2005).
Our view may also imply that the atmosphere was open for time intervals longer than those generally thought; assuming that (i) the whole-mantle degassing as a rate process, and (ii) the ratio of radiogenic isotopes ¹²⁹Xe/⁴⁰Ar best provides the chronological information on the atmosphere, it is estimated that the atmosphere was subject to hydrodynamic loss as long as 150 Myr after start of the solar system condensation.
DE: 1030 Geochemical cycles (0330)
DE: 1065 Major and trace element geochemistry
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005