Distribution of manganese between coexisting biotite and hornblende in plutonic rocks

The distribution of manganese between coexisting biotite and hornblende for 80 mineral pairs from igneous rocks of diverse provenance (including Southern California, Sierra Nevada, Boulder, and Boulder Creek batholiths and the Jemez Mountains volcanics) has been determined by neutron activation analysis. Data on the distribution ratio (Kd=MnhornblendeMnbiotite">Kd=MnhornblendeMnbiotite) indicate that an equilibrium distribution of Mn is closely approached, though not completely attained, in most samples from plutonic environments. Comparison of K_d values of mineral pairs with bulk chemical composition of host rocks reveals no correlation. Because initial crystallization temperatures vary with rock composition, the lack of correlation of composition with K_d suggests that the equilibrium distribution of Mn between biotite and hornblende reflects exchange at subsolidus temperatures rather than initial crystallization temperatures. The highest K_d values are for volcanic rocks, in which rapid quenching prevents subsolidus redistribution of Mn.

For sample pairs from the Southern California and Sierra Nevada batholiths there is a positive correlation of K_d with TiO₂ content of biotite. Though the evidence is not compelling, K_d may also correlate with the rate of cooling and/or the presence or absence of sphene in the rock.