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Loss of iron to gold capsules in rock-melting experiments

American Mineralogist

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Abstract

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 ??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 (~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 1n K' vs. 1/T relation (at an estimated log f(O)(2) 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 of f(O)(2) and sample to capsule mass ratios. At high f(O)(2) (NNO + 2), loss of Fe to gold is negligible (<2% relative) for a wide range of sample to capsule mass ratios. At an f(O)(2) 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(O)(2) (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(O)(2) conditions, be they buffered, imposed by the pressure media, or produced by intrinsically reduced (graphitic) starting materials.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Loss of iron to gold capsules in rock-melting experiments
Series title:
American Mineralogist
Volume
84
Issue:
10
Year Published:
1999
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
American Mineralogist
First page:
1521
Last page:
1527
Number of Pages:
7