Answers for Chapter 9: Trace Elements in Chemical Fractionation V.M. Goldschmidt proposed the following simplistic rules to qualitatively predict element distribution: A. Two ions with the same radius and valence should enter into solid solution in amounts proportional to their concentration. In other words, they should behave about the same. B. If two ions have a similar radius and the same valence, the smaller ion is preferentially incorporated into the solid as compared to the liquid. C. If two ions have a similar radius but different valence, the ion with the higher charge is more readily incorporated into the solid than the liquid. 1. Look at the periodic table and use Rules A and B to predict the general behavior of Rb in minerals and melts: to what element will it behave similarly? Will it enter first-forming minerals on Bowen’s Series, or remain in late melts? In which minerals will it concentrate? Rb will behave similarly to K, concentrating late melts, and in K-feldspar and micas. 1
2. Ni, in contrast to Rb, typically has a valence of +2. To what major elements will it most likely be similar? In which minerals is it likely to concentrate? Will in concentrate in late melts? Ni 2+ behaves similarly to Mg 2+ and Fe 2+ concentrating in early mafics and not in late melts. 3. Use the periodic table and Rule B to predict whether Fe 2+ or Mg 2+ will concentrate more in liquids than will coexisting mafic minerals. Can you support your contention with a binary phase diagram from Chapter 6? Mg 2+ is smaller than Fe 2+ , so should concentrate more in the solid phase. This is supported by the Fo-Fa diagram (Figure 6.10) in which the solidus is more Mg-rich than the liquidus at any temperature. 4. Use Rule C to predict whether Cr +3 and Ti +4 are preferred in solids as compared to liquids. High valence predicts they incorporate preferentially into solids. A more rigorous and quantitative approach is to actually measure the distribution of elements between minerals and coexisting liquids in melting experiments. From them we can define: (liquid) (solid) i i → (9.1) Always write the reaction before you analyze one. K D = X X i solid i liquid (9.2) distribution constant Where solid i X is the mole fraction of component i in the solid phase = n i /(n i + n j +n k …) K D = C C S L = D for traces (9.3) distribution coefficient or partition coefficient Where C S is the concentration of component i in the solid. 5. Use Table 9.1 (next page) to determine whether Rb is an incompatible or a compatible element. What about Ba? Ni? Cr? Eu? Rb is highly incompatible (excluded from olivine, pyroxenes, even plagioclase). Ni and Cr are compatible. Eu 3+ is incompatible, so is Eu 2+ (but it concentrates in plagioclase). 6.
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