AGOH AS A H₂O DELIVERY VECTOR FOR VAPOR-FLUXED MELTING EXPERIMENTS

A new method of adding precisely measured amounts of H₂O to vapor-fluxed melting experiments using silver hydroxide (AgOH) is presented here. AgOH is made via the exchange reaction:

NaOH (aq) + AgNO₃ (aq) → NaNO₃ (aq) + AgOH (s) (1)

At high temperatures, AgOH breaks down according to the reaction:

2AgOH (s) → 2Ag (m) + H₂O (v) + ½O₂ (v) (2)

releasing H₂O and O₂ into the system. The products in this reaction are either left behind as inert byproducts, as in the case of Ag, leave the system entirely by diffusion through the Au capsules, like O₂, or react with the starting material, like H₂O. Microprobe analysis of Fe-bearing phases (biotite, oxides) in finished run products show that Fe in these minerals is Fe²⁺, indicating that the O₂ released in reaction (2) has a minimal effect on f_O2 conditions. In piston-cylinder experiments designed to determine the melting conditions of a migmatitic gneiss, melting did not occur in vapor-absent experiments until 950°C (P=0.8 GPa). By comparison, melting began in experiments that contained added AgOH at 775°C (P=0.8 GPa). As the amount of AgOH increased in the experiments, corresponding to additions of 1 - 5 wt% H₂O, the amount of melt produced also increased. Adding H₂O to vapor-fluxed melting experiments via AgOH avoids the complications of adding small amounts of free water to capsules as well as the difficulties in ensuring that all the water remains in the capsule during welding. The amount of H₂O produced by the dehydroxylation of AgOH in piston-cylinder experiments is consistent, reproducible and within error of standard methods of H₂O addition to experiments.