MODELS FOR THE DISTRIBUTION OF TERRESTRIAL NOBLE GASES AND EVOLUTION OF THE ATMOSPHERE

Abstract

Noble gases provide unique clues to the structure of the Earth and the degassing of volatiles into the atmosphere. Since the noble gases are highly depleted in the Earth, their isotopic compositions are prone to substantial changes due to radiogenic additions, even from scarce parent elements and low-yield nuclear processes. On a global scale, noble gas isotopic variations reflect planetary differentiation processes that generate fractionations between these volatiles and parent elements. It has long been recognized that the atmosphere is not primary (i.e., solar) and so is not simply a remnant of the solar nebula. From similarities with volcanic emanations, a secondary origin of the atmosphere was proposed, that is, atmospheric volatiles are derived from degassing of the solid Earth (Brown 1949; Suess 1949; Rubey 1951). The characteristics of the atmosphere therefore reflect the acquisition of volatiles by the solid Earth during formation (see Pepin and Porcelli 2002, this volume), as well as the history of degassing from the mantle. The precise connection between volatiles now emanating from the Earth and the long-term evolution of the atmosphere are key subjects of modeling efforts. Major advances in understanding the behavior of terrestrial volatiles have been made based upon observations on the characteristics of noble gases that remain within the Earth. Various models seek to define different components and reservoirs in the planetary interior, how materials are exchanged between them, and how the noble gases are progressively transferred to the atmosphere. These models are evaluated here after a review of the available constraints on noble gas distributions and fluxes. Note that the crust is largely ignored here, because it is not a major noble gas reservoir because of reworking and thus extensive degassing (Ballentine and Burnard 2002, this volume). In this review, emphasis is placed on the specific …