Pd-oxide equilibration: a new experimental method for the direct determination of oxide activities in melts and minerals

Abstract

We have developed a new technique for the experimental determination of the activities of oxide components in melts and minerals using the equilibrium between Pd alloy, oxygen, and the oxide component in the sample of interest. If a melt or mineral sample is equilibrated with Pd metal at fixed P, T, and f _{O 2}, a small amount of each constituent oxide will reduce to metal and dissolve into the Pd, forming an alloy. Due to the extraordinary stability of dilute alloys of Pd with Mg, Al, and Si, these metals dissolve into the Pd in amounts easily measured with the electron microprobe at f _{O 2} s that can be achieved with conventional gas-mixing techniques. We determined the activity-composition relations for Pd−Mg, −Al, and −Si alloys by equilibrating Pd at fixed f _{O 2}and T with periclase, corundum, and cristobalite (a _oxide≡1). Because Mg, Al, and Si have constant activity coefficients in Pd at low concentrations, the activity of the oxide of each metal is a simple function of the ratio of the concentration of the metal in Pd in equilibrium with the sample to that in Pd in equilibrium with the pure oxide. Therefore, if Pd plus a melt or mineral and Pd plus pure oxide standards are equilibrated simultaneously at fixed T and f _{O 2}, the precision of the analytical technique is the major limitation on the determination of oxide activities. We used Pd-oxide equilibration to explore activities in silicate melts analogous to Type B Ca−Al-rich inclusions (CAIs) from carbonaceous chondrites; the measured activities deviate systematically from model valves but agree to within 1–30%. The activities imply that Type B CAIs did not condense as liquids from a gas of solar composition, and that only very aluminous compositions are potential liquid condensates from the solar nebula. We also used Pd-oxide equilibration to determine the free energy of formation from the oxides, ΔG ^/O _f , of the spinel end-member MgAl₂O₄ at 1150 to 1400°C to a precision of 2–19% (1∂). Because the technique reflects equilibration at high temperature, the ΔG ^/O _{f s} accurately represent the mineral with equilibrium Mg−Al disorder at temperature, a feature not true of drop calorimetric results because of partial reordering during quenching. Our results indicate more negative ΔG ^Emphasis>/O _f and hence higher entropy of formation, ΔS ^Emphasis>/O _f , than given in most compilations of thermodynamic data for spinel.