INDIUM IN FERROMAGNESIAN MINERALS: AN EXPERIMENTAL STUDY

Indium is widely used in technology today, with applications to smart phone screens, LCD displays, infrared cameras, and solar cells. Yet, little is known about its behavior in the felsic magmas related to indium-bearing vein deposits, a primary source of indium.

We are performing experiments on the partitioning of indium between ferromagnesian phases and rhyolitic melts to constrain the behavior of indium in magmatic systems. Starting materials consist of the assemblage sanidine + magnetite + enstatite + aqueous chloride solution (0.5 M KCl, 0.5 M NaCl, 0.01 M HCl) + biotite seed crystal + Bishop Tuff (melt) and an alloy phase (In₅Au₉₅). Experimental charges are contained in sealed gold capsules and placed in René 41 cold-seal vessels. Run conditions are 750° C or 800° C and 1000 bars with durations ranging from 1 to 8 weeks (n=11). The experiments are designed so that f_O2 can be determined given the presence of biotite, k-spar, and magnetite at a given f_H2O, (following Czamanske and Wones, 1973). The partition coefficient for indium between biotite and melt (D^bt/melt) varies as a function of biotite composition (X_Ann), and therefore f_O2 (by way of the low-variance mineral assemblage)_. Our tentative results show that: for f_O2 between NNO and NNO+1 and X_Ann of ~0.3, D^bt/melt= 3.7 ± 1.8 (1σ); and, for f_O2 between NNO+1 and ~NNO+2 and X_Ann of ~0.2, D^bt/melt= 10 ± 3.

Biotite is a common phase in felsic igneous rocks and is on the order of ~5 wt % in granites. Given the D^bt/melt determined in this study, calculated bulk Ds are on the order of 0.2 and 0.54 (for the effect of biotite alone) at lower and higher f_O2, respectively. By using these bulk D values and a modified Rayleigh fractionation equation, the proportion of indium sequestered by biotite prior to water saturation of a magma is calculated to be ~ 8% and 20%, respectively, at an initial water concentration of 4% and water concentration at saturation of 6%. The proportion of sequestered indium increases for lower initial water concentrations of the magma. For magmas with initial water concentrations of 1% and water saturation at 6%, the proportion of indium sequestered is 30% (low f_O2) and 62% (high f_O2). These preliminary results suggest that the bulk partition coefficient of biotite can account for wide variations in the proportion of indium incorporated into hypersolidus crystalline phases.