Excess Argon in Submarine Basalts and an Earth-Atmosphere Evolution Model

Abstract

PAIRS of isotopes such as 206Pb vs. 204Pb or 87Sr vs. 86Sr, in which one of the pair is radiogenic, have been used successfully to construct an evolutionary model of a mantle-crust system1–3. In these models, the isotopic ratios in basalts are generally considered to represent the values in the mantle, whereas the crustal values are estimated from analyses of representative surface rocks. In a similar way, the 40Ar/36Ar ratio may be used to trace the evolutionary history of the Earth-atmosphere system, because degassing processes from the Earth are of major importance in atmospheric evolution. Because of uncertainty in the argon isotopic ratios in the mantle, no such attempt has been made so far. Here we show that determination of excess argon in rock samples gives information on the isotopic ratios in the mantle and a reasonable Earth-atmosphere evolution model may be constructed from these data.