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. H2O‐saturated solidus of rock is critical to the debate. Unfortunately, there is a significant discrepancy in H2O‐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 H2O‐saturated solidus of rock systems by monitoring the electrical conductivity of the system in situ. A mixture of albite and >10 wt% H2O 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 H2O‐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 H2O‐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 H2O‐saturated solidus of a variety of rock systems and shed light on subduction zone processes. Plain Language Summary: To understand petrological processes at subduction zones, it is important to know at what temperature a rock starts to melt with plenty of water available (i.e., the H2O‐saturated solidus). However, in the products from high‐pressure quench experiments, it is often difficult to tell whether the rock system has melted or not. Not surprisingly, there are significant discrepancies between previous studies on the location of H2O‐saturated solidus, especially for peridotite. By using the albite‐H2O system as a test system, we show that melting can be recognized in situ by an electrical conductivity jump, although the jump becomes smaller toward high pressure. The solidus temperatures determined by our approach are in good agreement with widely accepted values in literature. With further improvement, this new approach can be used to solve the discrepancy over H2O‐saturated solidus of different rock systems and shed light on subduction zone processes. Key Points: A new in situ method of measuring H2O‐saturated solidus is developedMelting of albite corresponds to a jump of electrical conductivityThe amplitude of electrical conductivity jump reduces with pressure [ABSTRACT FROM AUTHOR]