Radiogenic isotope fractionation and planetary differentiation

Another example is provided by the chemical fractionation of tungsten into planetary cores. Tungsten has a short-lived radioactive isotope, W, which decays into Hf. Tungsten is siderophile and hafnium is lithophile. Consequently, the daughter isotope, 182Hf, will be found either in the core or the mantle depending on how quickly metal fractionation (core formation) occurred relative to the rate of decay. The Hf- W system is used to date core formation on planetary bodies. We will discuss the details of using radioactive isotopes as chronometers in Chapters 8 and 9. 

All of the bodies in the solar system formed from the same mixture of gas and dust inherited from the Sun s parent molecular cloud. The composition of the dust is best approximated by Cl chondrites. The current compositions of the bodies in our solar system came about because various chemical and physical processes fractionated the elements and isotopes in that initial composition. Understanding how and why elements and isotopes fractionate is a central theme of cosmochemistry. It is easy to visualize fractionations using certain kinds of diagrams that compare elements and isotopes with different chemical characteristics. 

We have seen that elements can be separated based on their volatility, either through gas-solid or gas-liquid reactions. There are many types of reactions that form a continuum between equilibrium condensation (or its inverse, evaporation) on the one hand and purely kinetically controlled reactions, such as Rayleigh distillation, on the other. In some cases, isotopic fractionation can assist in identifying the processes involved. 

Common igneous processes (partial melting and fractional crystallization) lead to element fractionations. Incompatible elements tend to be concentrated in melts and compatible elements in solids. Separation of partial melts from residual crystals as the melts ascend to higher levels, or accumulation of early-formed crystals from melts, ultimately produces rocks with compositions different from the starting materials. These processes account for the fractionations seen in differentiated meteorites and planetary samples. 

Next we will examine unstable isotopes in some detail. The chronologic information they provide, coupled with knowledge of cosmochemical and geochemical fractionations, are powerful tools in understanding nebular and planetary processes. 

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