Atmospheric ~(38)Ar/~(36)Ar in the mantle: Implications for the nature of the terrestrial parent bodies

摘要

Analyses of argon in basaltic glasses from mid-oceanic ridges and oceanic islands show that the ~(38)Ar/~(36)Ar isotopic ratio in the mantle does not differ from the atmospheric value and is different from the solar value. This contrasts with neon isotope systematics that shows solar-like, non-atmospheric, values in mantle-derived rocks. The air-like argon isotopic ratio for the Earth's mantle is not due to air contamination since they are associated with high ~(20)Ne/~(22)Ne ratios (> 12). We present new He, Ne, and Ar data for a suite of glasses from ocean ridges and ocean islands, with the goal of improving the accuracy of the mantle ~(38)Ar/~(36)Ar determination. The ~(20)Ne/~(22)Ne-~(40)Ar/~(36)Ar correlation observed in these samples allows us to give a well-constrained mantle ~(38)Ar/~(36)Ar ratio, which is clearly air-like. The Ne-Ar systematics in the mantle suggest that the Ne and Ar isotopic compositions may have been probably acquired by implantation of solar wind in the Earth's parent bodies rather than by solubilization in a magma ocean from the solar nebula or a primordial atmosphere. Indeed, a scenario of solar wind implantation with mass fractionation (as shown for Genesis targets) coupled with sputtering, is able to produce the Ne and Ar isotopic ratios observed in the Earth's mantle and in gas-rich meteorites. However, this scenario fails to explain the Kr and Xe isotopic signatures and a different origin for the Kr and Xe is required.