THE EVOLUTION OF TERRESTRIAL VOLATILES: A VIEW FROM HELIUM, NEON, ARGON AND NITROGEN ISOTOPE MODELLING

Abstract

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 reservoirs. The following processes appear to have played an important role in the history of terrestrial volatiles. During accretion, impact degassing could have released ≈95% of the initial rare gas abundances, which were quite similar to those typical of solar wind-implanted gases. After the major phase of accretion and core segregation (4.50 Ga ago), some parts of the upper mantle were partially melted by giant impact(s) and experienced vigorous convection and solubility-controlled degassing. More than 99% of the volatile species initially presented in the upper mantle–atmosphere reservoirs were lost during this period (from 4.50 to 4.30 Ga ago). This loss was accompanied by elemental and isotopic fractionation of residual atmospheric constituents. The atmosphere became retentive for Xe at 4.40 Ga but degassing, loss and fractionation of lighter rare gases and nitrogen might have taken place some time afterwards. Therefore each gas might have undergone fractionation to various extent at different times. After closure of the atmosphere (at ≈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, between ∼1% (4.3 Ga ago) and ∼0.2% (at present) of their total amount in the lower mantle per Ga. The post-atmosphere closure flux of liquid silicates from the upper mantle, analogue to the present-day MOR flux of basaltic melts, decreased by a factor of ∼100, from ∼5·1018 (4.3 Ga ago) to 6·1016 g a−1 (at present). The ratio of 36Ar(now)/36Ar(4.3 Ga)um∼10−4 illustrates the total degree of upper mantle degassing yielded by this flux. This rate of degassing corresponds to a present-day ratio of 40Ar/36Arum>106, if no fluxes from the lower mantle and the crustal–atmospheric reservoirs had operated, and the ratios of radiogenic over primordial species could have been higher in the past than those at present. The model postulates that nitrogen trapped in the Earth–Atmosphere system was initially depleted in 15N relative to present-day atmospheric composition (ATM). Atmospheric escape enriched the ancient atmosphere in 15N, resulting in a δ15N isotopic composition of +2.5 parts per mil (ATM) 4 Ga ago. Subsequent degassing of mantle nitrogen 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 primordial rare gases and nitrogen abundances in the upper mantle approximately at a steady state. Decays of parent radioactive elements, Uum, Thum, and Kum, contribute radiogenic nuclides. Recycling fluxes of (sub)surface materials into the upper mantle transfer surface volatiles, ∼6% of surface-derived (atmosphere+sediments) N and ∼0.03% of atmospheric 36Arah per Ga. Consequently, the isotopic composition of mantle nitrogen varied from its initial δ15N value 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–Atmosphere system is therefore partial re-homogenisation of N-isotopes, but this process was not efficient enough to have erased early heterogeneity. Thus the present contribution proposes early formation of the Earth's atmosphere by a combination of degassing, dissipation, and fractionation processes. Primordial rare gases and nitrogen were set in this reservoir around 4.3 Ga ago and further mantle outgassing has contributed less than 3% of these species since that time.