ACCRETION AND EARLY DEGASSING OF THE EARTH - CONSTRAINTS FROM PU-U-I-XE ISOTOPIC SYSTEMATICS

A review of problems related to Xe isotopic abundances in meteorites a nd terrestrial materials leads to four postulates which should be take n into account to build a model of the Earth's accretion and early evo lution. 1. The pre-planetary accretion time scale was shorter than the I-129 half-life, 17 Ma, so the initial ratio of I-129/I-127 had not b een decreased considerably when planetary accretion started; therefore , this must also be the case for the Pu-244 abundance. 2. The initial relative abundance of involatile refractory Pu-244 in proto-planetary materials should be the same as in chondrites, that is, Pu-244/U-238 = 0.0068; this value corresponds to initial Pu-244 congruent-to 0.30 pp b in the bulk silicate earth. In contrast, I is a highly volatile elem ent; its initial abundance, accretion history and even the present-day mean concentrations in principal terrestrial reservoirs are poorly kn own. 3.There is much less fission Xe in the upper mantle, crust, and a tmosphere than is predictable from the fission of Pu-244 (Xe(Pu)) base d on the above argument. Therefore, Xe(Pu) has been mainly released fr om these reservoirs. 4. A mechanism for Xe(Pu) escape from the complem entary upper mantle-crust-atmosphere reservoirs, for example, atmosphe ric escape via collisions of a growing Earth with large embryos and/or hydrodynamic hydrogen flux, etc., operated during the Earth's accreti on. These postulates have been used as a background for a balance mode l of homogeneous Earth accretion which envisages: growth of the Earth due to accumulation of planetesimals; fractionation inside the Earth a nd segregation of the core; degassing via collision and fractionation; and escape of volatiles from the atmosphere. During the post-accretio n terrestrial history, the processes described by the model are contin uous fractionation, degassing and recycling of the upper mantle and cr ust. The lower mantle is considered as an isolated reservoir. Dependin g on the scenario invoked, the accretion time scale varies within the limits of 50 - 200 Ma. In the light of recent experimental data, the l atter value is inferred to the most realistic version which explains a high Xe(U)/Xe(Pu) ratio in the upper mantle. Contrary to previous sug gestions, the I-129-Xe-129 subsystem is considered to be meaningless w ith regard to the terrestrial accretion time scale. The terrestrial in ventory of Xe-129(I) is controlled by the initial abundance of volatil e elements (including I and Xe) in proto-terrestrial materials and the subsequent degassing history of the Earth. The residence time of a vo latile element (eg., Xe) in the bulk mantle (bm) during accretion, [t( Xe)bm], is approximated by the ratio of [t(Xe)bm] almost-equal-to m(bm )(t)/phi(bm,mf) less-than-or-equal-to 10 Ma, where m(bm)(t) is the man tle mass, and phi(bm,mf) is the rate of metal/silicate fractionation, which provided segregation of the core; phi(bm,mf) is determined by in volatile siderophile element abundances in the upper mantle. This rela tionship implies a link between the abundance of involatile siderophil e and volatile incompatible elements. A short [t(Xe)bm] reflects a hig h degassing rate due to extremely high phi(bm,mf) almost-equal-to 10(2 0) g/year. A small ratio of the atmospheric amount of Xe over the tota l amount of this gas in prototerrestrial materials, less-than-or-equal -to 0.01, is in accord with the process of Xe escape and fractionation in the primary Earth atmosphere.