Figure 5. Rb (ppm) vs SiO2 (wt.%) for Crater Peak, Mt. Spurr summit dome, and ancestral Mt. Spurr lavas. The difference in SiO2 between 1953 and 1992 lavas requires0% crystal fractionation and predicts0% increase in Rb. The lack of Rb increase suggests the two lavas are not related by crystal fractionation.

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