You visit a young volcanic area and find volcanoes erupting magmas that range in composition from basalt to dacite. There are two hypotheses for how this range of magmas arose:
1. Crystal fractionation from a parent basaltic magma.
2. Mixing of basaltic magma with rhyolite magma derived by partial melting of Precambrian granite.
Devise a geochemical test, using isotopic data, for evaluating these hypotheses. What does each predict about isotopic variability in the suite?