Accretion and early degassing of the Earth: Constraints from Pu-U-I-Xe isotopic systematics

Details Accretion and early degassing of the Earth: Constraints from Pu-U-I-Xe isotopic systematics Accretion and early degassing of the Earth: Constraints from Pu-U-I-Xe isotopic systematics

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..609a&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 28, no. 5, p. 609-621

Computer Science

5

Degassing, Earth (Planet), Earth Atmosphere, Earth Mantle, Meteorites, Nuclear Reactions, Planetary Evolution, Solar Corona, Solar Ponds (Heat Storage), Atomic Weights, Deposition, Iodine Isotopes, Plutonium 244, Radiogenic Materials, Rare Gases, Uranium, Xenon Isotopes

Scientific paper

A review of problems related to Xe isotopic abundances in meteorites and terrestrial materials leads to four postulates which should be taken into account to build a model of the Earth's accretion and early evolution. 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 been 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 approximately = 0.30 ppb in the bulk silicate earth. In contrast, I is a highly volatile element; its initial abundance, accretion history and even the present-day mean concentrations in principal terrestrial reservoirs are poorly known. 3. There is much less fission Xe in the upper mantle, crust, and atmosphere than is predictable from the fission of Pu-244 (Xe(Pu)) based on the above argument. Therefore, Xe(Pu) has been mainly released from these reservoirs. 4. A mechanism for Xe(Pu) escape from the complementary upper mantle-crust-atmosphere reservoirs, for example, atmospheric escape via collisions of a growing Earth with large embryos and/or hydrodynamic hydrogen flux, etc., operated during the Earth's accretion. These postulates have been used as a background for a balance model of homogeneous Earth accretion which envisages: growth of the Earth due to accumulation of planetsimals; fractionation inside the Earth and segregation of the core; degassing via collision and fractionation; and escape of volatiles from the atmosphere. During the post-accretion terrestrial history, the processes described by the model are continuous fractionation, degassing and recycling of the upper mantle and crust. The lower mantle is considered as an isolated reservoir.

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