AN OVERVIEW OF NOBLE GAS GEOCHEMISTRY AND COSMOCHEMISTRY

Abstract

A wealth of fundamental information regarding the Earth and solar system is based upon observations of the highly volatile elements He, Ne, Ar, Kr, and Xe. At first, this may seem surprising, considering that these elements are generally thought to reside almost entirely in the atmosphere, and so are considered strongly ‘atmophile.’ However, increasingly sophisticated analytical techniques have provided the means for precisely measuring their abundances in a wide range of geological and cosmochemical materials. Fittingly, these elements are known collectively as the rare gases, reflecting their general scarcity in geological materials. It is this feature that continues to provide challenges for analysts. These elements are also the noble gases, in tribute to their disdain for engaging in chemical consort with other species. Such behavior has been responsible for the early difficulties in their detection, and facilitates their continuing migration to the atmosphere. However, others refer to these as ‘the inert gases,’ which seems to imply that their behavior is dictated by a lack of interest in chemical reaction, a deficiency in chemical drive. Overall, the choice of appellation depends upon whether scarcity, nobility, or inertness is considered the most important characteristic. Regardless of their motivations, these noble gases can be profitably considered together, because physical and chemical properties vary systematically with atomic weight. However, much of the utility of noble gases is based on the widespread variations in their isotopic compositions. This is related to their overall depletion, which has made these elements vulnerable to isotopic modification from nuclear processes involving relatively more abundant parent elements. The wide applicability of noble gas systematics is due to the range of such processes. In cosmochemistry, fundamental contributions have been made to understanding the sources and distributions of volatiles throughout the solar system, to identifying the preservation of nucleosynthetic anomalies in meteorites, …