NOBLE GAS CONSTRAINTS ON THE EVOLUTION OF THE ATMOSPHERE-MANTLE SYSTEM - CONSTRAINTS ON ATMOSPHERE AND MANTLE EVOLUTION

Abstract

We present a model on the evolution of atmosphere-mantle system assuming a significant degassing from the less depleted mantle, based on the recent results of the He-Ar systematics proposed by Matsuda and Marty (1995). The degassing fluxes of noble gases are represented by the concentrations in the mass flow, which follows the model of Porcelli and Wasserburg (1995a, Porcelli and Wasserburg 1995b). However, we have not assumed the steady-state, and have assumed the mass transfer as an exponential function of time. The degassed amount from the less depleted mantle is related to the mixed amount of mass between the less depleted mantle and the depleted mantle. The amount of mixing is constrained from the K abundances in each mantle reservoir. Consequently, the degassed amount from the less depleted mantle is also constrained.The following conclusions are obtained from this model. The initial elemental ratio 3He/36Ar is likely to be in cosmic abundances, and the initial concentration of 36Ar in the Earth is constrained to be ~1 x 1012 atoms/g. The present fraction of 36Ar in the depleted mantle reservoir is calculated to be at most 2.4 x 10-3 of the initial inventory. The 40Ar/36Ar ratio in this reservoir is estimated to be less than (1.3-1.4) x 105. The less depleted mantle reservoir also degassed and the present amount of 36Ar is calculated to be at most 1.3 x 10-1 of the initial inventory. The 40Ar/36Ar ratio in the less depleted mantle reservoir is estimated to be less than 3.7 x 104.To explain the difference between the neon isotopic ratios in the atmosphere and those observed in the mantle rocks, isotopic fractionation during the escape from the atmosphere and/or significant contribution of late accreted materials possessing planetary Ne is necessary. In our model, Ne degassing from both mantle reservoirs would have provided at least 50-90 times the amount of 22Ne in the present atmosphere. Therefore, the fraction supplied by late-accreted material would be negligible, which is contrary to the model proposed by Porcelli and Wasserburg (1995b). We conclude that isotopic fractionation model during escape from the atmosphere provides the best fit to the observations.