The basalts of the Black Rock Desert region in Utah have Sr87/Sr86 ratios that vary from 0.7044 to 0.7059. Although this variation may be explained by derivation from inhomogenous mantle, it is also broadly compatible with a fractionation-assimilation model proposed for these basalts. Basalts of the Ice Spring field show an increase in Sr87/Sr86 with decreasing Rb and Rb/Sr as a function of eruptive age. Such a trend may be explained by mixing of magmas having different differentiation-assimilation histories.

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