Preferential formation of the atmosphere–sialic crust system from the upper mantle

Abstract

IN the past decade, extensive studies of the rare gas content of mantle derived rocks have led to several models of Earth degassing, all of which assume that the argon of the atmosphere degassed from the entire mantle. We give evidence here to support the concept of concurrent transfer of elements to the atmosphere and sialic crust preferentially from the upper mantle. This concept accounts for the following observations: (1) the ⁴⁰Ar concentrations in the rapidly quenched glassy margins of deep sea pillow basalts are too low for many estimates of the K concentration of the mantle^1,2; (2) the ⁴⁰Ar/³⁶Ar ratio of rocks derived from the deep mantle is similar to the atmospheric ⁴⁰Ar/³⁶Ar ratio^3–7; (3) the upper mantle under oceanic ridges is depleted in K and other large-ion-lithophile elements. A schematic representation of the upper mantle depletion process is given in Fig. 1 which shows that partial melting associated with spreading ridges and Benioff zones exclusively in the upper mantle is responsible for the accretion of continents and associated degassing to the atmosphere.