Formation of the 15 Å phase as the most expanded hydrated mineral in cold subduction zone

Abstract

Talc, as an important class of clay minerals constituting subducting oceanic crust, has long been known to undergo interlayer expansion by similar to 6% to contain net similar to 13 wt.% water into the 'so-called' 10 angstrom phase. Although subduction fluid is mildly alkaline and includes various salts and other dissolved species, its effect on the stability of subducting minerals has not yet been considered. Here, we report that subducting talc, when exposed to alkaline salty water conditions, breaks down to form a super-hydrated 15 angstrom phase at similar to 3.0 GPa and similar to 350 degrees C, corresponding to a depth of similar to 90-95 km along a cold subduction geotherm. The 15 angstrom phase remains stable down to similar to 125 km depth, where it transforms into the previously known 10 angstrom phase. Our combined experimental and computational results show that the super-hydrated 15 angstrom phase contains net similar to 31 wt.% water through interlayer expansion by similar to 60%. Our work thus demonstrates mineral transformation under more realistic subduction environments, which calls for reevaluation of subduction-related geochemistry and seismicity as well as water transportation into the deep Earth.