Interpret the data

Causes of the spatial variations

• Question 7. What are some possible or likely causes for the non-random spatial distribution of the volcanic rocks in the western U.S., particularly the concentrations of datapoints that are arrayed in a linear or curvilinear fashion? (Use your general knowledge of plate tectonics and the geology of the western U.S. to answer this question).

Causes of the regional isotopic variations

• Question 8. Let's think about what the regional Sr isotope variation may be telling us about the mantle sources that produced these magmas. What characteristics (of the Earth's mantle) would lead to partial melts with high ⁸⁷Sr/⁸⁶Sr? What about low ⁸⁷Sr/⁸⁶Sr?

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  Look at the equation in Part 2.

• Question 9. Given your response for the previous question, how would you interpret the regional differences in Sr isotopic composition in the western U.S.?

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  Again, the equation in Part 2 should be helpful.

• Question 10. By now, you should be realizing that the age of the mantle source is an important factor which controls the ⁸⁷Sr/⁸⁶Sr of mafic magmas. Assuming they started out with similar Rb/Sr ratios, what is the difference in age (Δt) between the mantle source that produced the cluster of high ⁸⁷Sr/⁸⁶Sr mafic rocks in the western U.S. and the mantle source that produced the cluster of low ⁸⁷Sr/⁸⁶Sr mafic rocks? Using your results from Question 6, and the equation in Part 2, you can calculate this, but your result for Δt will be dependent on the value you choose for ⁸⁷Rb/⁸⁷Sr. Calculate Δt assuming ⁸⁷Rb/⁸⁷Sr = 0.15 (an appropriate value for the mantle).