Gold is used widely for capsules in high-temperature rock-melting studies b
ecause it is generally thought to absorb negligible Fe from silicate sample
s. However, we observed significant losses of Fe from fluid-absent melting
experiments on hornblende gabbros at 800-975 degrees C and 8 kbar, using st
andard 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 capsule
s 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 equilib
rium 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 sa
mple 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 ratio
s. At an f(o2) of NNO, Fe loss can be kept to <10% relative by using a samp
le to capsule mass ratio of 0.2 or greater. At low f(o2) (QFM-1), presatura
ting the Au with Fe would be necessary to ensure that Fe losses remained <1
0% relative. Fe loss can compromise experimental results for small samples
run at low f(o2) conditions, be they buffered, imposed by the pressure medi
a, or produced by intrinsically reduced (graphitic) starting materials.