Oxygen Isotopes in the Solar System

Abstract

The oxygen three-isotope system has major advantages over the two-isotope systems of hydrogen, carbon, and nitrogen because different fractionation laws govern intraplanetary and interplanetary processes. This permits discriminating between solar nebular processes and parent-body processes. Oxygen isotopes also serve as a sensitive natural tracer for meteorite classification. 1. Isotopic Composition of the Sun and the Solar Nebula Oxygen has three stable isotopes with mean terrestrial abundance ratios: 17 0/ 16 0 = 1/2700, 18 0/ 16 0 = 1/490. These ratios are variable in natural materials due to a variety of physical and chemical processes, ranging from stellar nucleosynthesis to ordinary mass-dependent chemical fractionation. As a natural isotopic tracer, oxygen has an advantage over other light elements, such as hydrogen, carbon, and nitrogen, in that the combined variation of two isotope ratios (17 0/ 16 0 and 18 0/ 16 0) helps to identify the underlying process. Chemical and physical processes within an isolated planetary body, such as Earth, Moon, and Mars, almost always obey a simple mass-dependent relationship between variations in Meteorite studies have shown that oxygen isotope abundance variations between solar system bodies are governed primarily by a different systematic relationship: In this delta-notation, isotope ratios are given relative to a standard reservoir. For solar system studies, the ideal standard would be the Sun itself, representing the reservoir from which planetary bodies were formed. Unfortunately the solar isotopic composition is not yet known with sufficient accuracy for that purpose. Therefore , the solar composition must be inferred from meteorite observations by a boot-strap process with inherent model-dependent ambiguities. The cosmic abundance of oxygen is several times greater than the sum of the abundances of elements with which it can form solid compounds (mostly Mg, Si,