Loss of iron to gold capsules in rock-melting experiments

Gold is used widely for capsules in high-temperature rock-melting studies because it is generally thought to absorb negligible Fe from silicate samples. However, we observed significant losses of Fe from fluid-absent melting experiments on hornblende gabbros at 800-975 degrees C and 8 kbar, using standard piston-cylinder techniques. The extent of Fe loss from the sample is dependent on the relative masses of the sample and the capsule. Low sample to capsule mass ratios (similar to 0.04) lead to the highest Fe losses (32-49% relative). Concentrations of Fe in silicate melt and used gold capsules define an apparent equilibrium constant (K') that follows a linear In K' vs. 1/T relation (at an estimated log f(o2) of QFM-1). The apparent equilibrium constant is used to make limiting upper estimates on the amount of Fe that could be lost during rock-melting experiments for a range off,, and sample to capsule mass ratios. At high f(o2) (NNO + 2), loss of Fe to gold is negligible (<2% relative) for a wide range of sample to capsule mass ratios. At an f(o2) of NNO, Fe loss can be kept to <10% relative by using a sample to capsule mass ratio of 0.2 or greater. At low f(o2) (QFM-1), presaturating the Au with Fe would be necessary to ensure that Fe losses remained <10% relative. Fe loss can compromise experimental results for small samples run at low f(o2) conditions, be they buffered, imposed by the pressure media, or produced by intrinsically reduced (graphitic) starting materials.