THE EVOLUTION OF TERRESTRIAL VOLATILES - A VIEW FROM HELIUM, NEON, ARGON AND NITROGEN ISOTOPE MODELING

In this contribution, we have developed an evolutionary model in order to identify and quantify processes which were able to reproduce rare gas and nitrogen isotopic abundances in the main terrestrial reservoir s. The following processes appear to have played an important role in the history of terrestrial volatiles. During accretion, impact degassi ng could have released approximate to 95% of the initial rare gas abun dances, which were quite similar to those typical of solar wind-implan ted gases. After the major phase of accretion and core segregation (4. 50 Ga ago), some parts of the upper mantle were partially melted by gi ant impact(s) and experienced vigorous convection and solubility-contr olled degassing. More than 99% of the volatile species initially prese nted in the upper mantle-atmosphere reservoirs were lost during this p eriod (from 4.50 to 4.30 Ga ago). This loss was accompanied by element al and isotopic fractionation of residual atmospheric constituents. Th e atmosphere became retentive for Xe at 4.40 Ga but degassing, loss an d fractionation of lighter rare gases and nitrogen might have taken pl ace some time afterwards. Therefore each gas might have undergone frac tionation to various extent at different times. After closure of the a tmosphere (at approximate to 4.30 Ga) for all but the lightest (H, He) constituents, the lower mantle supplied the upper mantle with minute amounts of parent incompatible elements, rare gases and nitrogen, betw een similar to 1% (4.3 Ga ago) and similar to 0.2% (at present) of the ir total amount in the lower mantle per Ga. The post-atmosphere closur e flux of Liquid silicates from the upper mantle, analogue to the pres ent-day MOR flux of basaltic melts, decreased by a factor of similar t o 100, from similar to 5.10(18) (4.3 Ga ago) to 6.10(16) g a(-1) (at p resent). The ratio of Ar-36(now)/Ar-36(4.3 Ga)(um) similar to 10(-4) i llustrates the total degree of upper mantle degassing yielded by this flux. This rate of degassing corresponds to a present-day ratio of Ar- 40/Ar-36(um) > 10(6), if no fluxes from the lower mantle and the crust al-atmospheric reservoirs had operated, and the ratios of radiogenic o ver primordial species could have been higher in the past than those a t present. The model postulates that nitrogen trapped in the Earth-Atm osphere system was initially depleted in N-15 relative to present-day atmospheric composition (ATM). Atmospheric escape enriched the ancient atmosphere in N-15, resulting in a delta(15)N isotopic composition of +2.5 parts per mil (ATM) 4 Ga ago. Subsequent degassing of mantle nit rogen allowed this element to reach its present-day composition in air . Because the upper mantle is extremely depleted in volatile elements, their transfer from the lower mantle is sufficient to maintain primor dial rare gases and nitrogen abundances in the upper mantle approximat ely at a steady state. Decays of parent radioactive elements, U-um, Th -um, and K-um,, contribute radiogenic nuclides. Recycling fluxes of (s ub)surface materials into the upper mantle transfer surface volatiles, similar to 6% of surface-derived (atmosphere + sediments) N and simil ar to 0.03% of atmospheric Ar-36(ah), per Ga. Consequently, the isotop ic composition of mantle nitrogen varied from its initial delta(15)N v alue of - 30 parts per mil to its present-day upper mantle value of -5 parts per mil. The consequence of nitrogen recycling in the Earth-Atm osphere system is therefore partial re-homogenisation of N-isotopes, b ut this process was not efficient enough to have erased early heteroge neity. Thus the present contribution proposes early formation of the E arth's atmosphere by a combination of degassing, dissipation, and frac tionation processes. Primordial rare gases and nitrogen were set in th is reservoir around 4.3 Ga ago and further mantle outgassing has contr ibuted less than 3% of these species since that time. (C) 1998 Elsevie r Science B.V. All rights reserved.