Determination of the H₂O‐Saturated Solidus of Albite by Electrical Conductivity

In the study of subduction zone processes, there has been much debate on slab dehydration against slab melting, as well as on flux melting of the mantle wedge against metasomatism and a prolonged period of storage and heating before mantle melting. H 2 O‐saturated solidus of rock is critical to the debate. Unfortunately, there is a significant discrepancy in H 2 O‐saturated solidus between different experimental studies, mainly due to the difficulty in distinguishing hydrous melt from aqueous fluid in quenched experimental products. In this study, we developed a new approach to locating H 2 O‐saturated solidus of rock systems by monitoring the electrical conductivity of the system in situ. A mixture of albite and >10 wt% H 2 O was heated stepwise in the range of 0.35–1.7 GPa in a piston cylinder apparatus. A jump in electrical conductivity (up to 1.8 log units) was observed over a small temperature interval (10–20°C). The temperature corresponding to the electrical jump decreased from 790 ± 10°C at 0.35 GPa to 640 ± 10°C at 1.7 GPa, in good agreement with previously reported H 2 O‐saturated solidus of albite. Examination of the experimental products quenched immediately after observation of the electrical conductivity jump confirmed the occurrence of albite melting. Our experimental results suggest that the electrical conductivity jump serves as a good indicator for rock melting under H 2 O‐saturated conditions, and this in situ approach can overcome the ambiguity in phase identification in quench experiments. This new approach has the potential to solve the discrepancy over H 2 O‐saturated solidus of a variety of rock systems and shed light on subduction zone processes.