Practical #1
Practical 1: Modeling Trace Elements and Isotopes
Objective: To review the results of simple calculations that involve changes in trace elements and isotopes during fractional crystallization and magma mixing.
1a. In excel, calculate the abundances of Zr, Sr and Ni for a parental magma that undergoes fractional crystallization, given the following information. Use increments of F = 0.05, and cease the calculation at F = 0.05.
Note: Think about the formulas for olivine, clinopyroxene and plagioclase and the valencies or sizes of Zr, Sr, and Ni. Consider which elements would most likely fit into these mineral phases. Now look at D for each one. Notice anything?
Table 1: Mineral Data
phase / % in assemblage / DZr / DSr / DNiolivine / 40 / 0.06 / 0.001 / 12
clinopyroxene / 30 / 0.21 / 0.24 / 2.6
plagioclase / 30 / 0.001 / 3 / 0.06
Parental composition
Zr 170 ppm
Sr 600 ppm
Ni 200 ppm
1b. Why is the daughter concentration of Sr at F = 0.05 so close to the parent (F = 1)?
The daughter concentration of Sr at F=0.05 is so close to the parent because the bulk D for Sr is so close to 1.
1c. What assumption(s) is (are) made about this calculation for the range F = 1 à F = 0.05?
An assumption is made that this system is closed, therefore there is no contamination from outside sources which may or may not cause the results to vary.
2. Given the following changes in D (Table 2), hypothesize about how the compositions of the daughter products will change, compared to the example in question #1. Provide your reasoning.
The bulk D for Zr will be lower in this case, causing the concentration in the daughter to be much higher than before. For Sr, the bulk D will be higher causing the concentration in the daughter to be much lower than before. The bulk D for Ni will end up being much higher, causing the concentration to reach zero sooner than before.
Table 2: Mineral Data
phase / % in assemblage / Zr / Sr / Niolivine / 40 / 0 / 0.05 / 20
clinopyroxene / 30 / 0.05 / 0.5 / 4
plagioclase / 30 / 0 / 5 / 0.1
3. Repeat the calculations required in question #1 for data in Table 2. Do your calculations confirm your hypothesis from question #2?
4. For the data in Table 2, how would your results change if the phase assemblage were to include only plagioclase?
If the phase assemblage were to include only plagioclase the concentration of Zr would end up being higher, the concentration of Sr would be lower (possibly even zero) and the concentration of Ni would be much higher, considering that in both previous cases the bulk D for Ni was relatively large (causing Ni to fall to zero).
5. If the parental magma in these examples has 87Sr/86Sr = 0.7035, in a binary plot, illustrate the F vs. 87Sr/86Sr trend. Defend your answer.
6. Given the following data on end-member compositions, complete the table below.
Table 3: Geochemical Data for End-Members A and B
End-member / Zr / Sr / Ni / 87Sr/86SrA / 50 / 450 / 140 / 0.704
B / 250 / 35 / 10 / 0.709
Table 4: Concentrations of Zr, Sr and Ni in Mixed Magmas
Mixed magma / Zr / Sr / Ni / 87Sr/86Sr20% A / 210.00 / 118.00 / 36.00 / .70519
40 % A / 170.00 / 201.00 / 62.00 / .70452
55% A / 140.00 / 263.25 / 81.50 / .70430
75 % A / 100.00 / 346.25 / 107.50 / .70413
7. Sketch two binary plots, illustrating the Zr-Sr and Zr-Ni characteristics of the 4 mixed products. Label each point with the mixed magma “sample name” from the first column of Table 4.
8. What geochemical criteria can be used to identify/diagnose that magma mixing has been an important process in the evolution of this series of mixed magmas?
All values are intermediate between the two end member magmas.
9. Qualitatively describe the complications that may arise in the Zr-Sr plot if plagioclase were to crystallize from the mixed magmas represented by 55% and 75% A.
Sr is compatible with plagioclase, and if enough plagioclase were to crytallize during this mixing process it may cause the concentration of Sr to go down significantly. In the case of Zr, it is incompatible which would cause the concentration to go up in the magma. The Zr-Sr plot would no longer be linear, which would make it harder to obtain evidence for magma mixing.
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