Distribution of scandium between coexisting biotite and hornblende in igneous rocks

Scandium analyses of more than 90 pairs of coexisting biotite and hornblende from igneous rocks of various provinces (including Southern California, Boulder, Sierra Nevada, Boulder Creek batholiths and the Jemez Mountains volcanic rocks) indicate that the distribution ratio (K_d = Sc_hornblende/Sc_biotite) for most samples closely approached that of an equilibrium distribution. Median K_d values for the igneous samples range from 4.8 to 8.0, which are higher than similar values derived from published data on metamorphic samples and apparently not related to the mode of crystallization (volcanic, hypabyssal, or plutonic). A correlation between Kd and mafic index, (FeO + Fe₂O₃)/(FeO + Fe₂O₃ + MgO) X 100, of both the minerals and the host rock, and between K_d and SiO₂ content of the host rock, can be established only for the Southern California batholith samples. However, whether this correlation reflects temperature dependence, compositional dependence, or both, cannot be specified uniquely with present data. The present data also cast doubt on the validity of the so-called “scandium geothermometer.”