EMU Volume 16 – Chapter 17

Chapter 17. Kinetics of stable and radiogenic isotope exchange in geological and planetary processes

J. A. Van Orman and M. J. Krawczynski

Stable and radiogenic isotopes play a central role in the geosciences, due to their importance as geochemical tracers, thermometers, speedometers and chronometers. This chapter focuses on the importance of isotope transport and exchange in elucidating hightemperature petrogenetic processes. Stable isotopes are fractionated in several ways, including by temperature-dependent equilibrium partitioning between phases and by mass-dependent diffusion processes. The distribution of stable isotopes among a polyphase assemblage reflects a competition between equilibrium partitioning at interfaces between phases, and diffusive transport within the mineral interiors. Both of these processes are temperature-dependent: equilibrium partitioning becomes more pronounced and diffusion of isotopes becomes more sluggish as temperature decreases. This provides the basis for a cooling speedometer, provided that the equilibrium stable isotope fractionation factors and diffusion coefficients are known. Stable isotopes can also be fractionated by chemical diffusion within a phase, due to the dependence of diffusivity on isotope mass. This diffusion-induced isotopic fractionation provides a rigorous basis for distinguishing chemical zoning profiles produced by diffusion from similar chemical zoning produced by different processes. The magnitude of the isotopic mass dependence also provides information that can help to elucidate the diffusion mechanism.

Radiogenic isotopes have a number of important uses, from tracing chemical heterogeneity in Earth’s mantle to providing absolute ages for geological events. Radiogenic isotopes have long been used to provide constraints on cooling rates, based on knowledge of the diffusivity as a function of temperature. The most commonly used models assume: (1) an infinite sink for radiogenic daughters, and (2) a simple Arrhenian dependence of the diffusion coefficient. These and other simplifying assumptions are not always met, and in these situations it is necessary to model the isotope exchange process numerically. We focus here on numerical simulations of simultaneous radioactive decay and diffusive exchange in the short-lived 107Pd-107Ag system in iron meteorites.

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