Direct evidence for decomposition of antigorite under shock loading

Detailed wave profiles of antigorite (a serpentine mineral) under plate‐impact shock loading have been measured to a pressure of 131 GPa in order to understand its dynamic behavior because serpentine is present in pristine meteorites as well as in the Earth mantle. All the profiles indicate single wave structures, and a sudden decrease in density was detected at ∼60 GPa with increasing pressure when shock‐loaded for a long duration (∼0.6 μ s) using a thick flyer. Such a drop in density also was observed during recompression by a high‐impedance window material (LiF). Although exothermic decomposition is generally considered to be fast, the decomposition of antigorite under shock loading requires a reaction time so that the Hugoniot may represent a metastable state at which stable phase cannot appear in a timely way. Based on these observations, antigorite decomposes exothermically into an assemblage of either brucite + stishovite + periclase or brucite + perovskite above a shock pressure of 60 GPa, but does not dehydrate endothermically into assemblages with water fluid. The observed dynamic behavior of serpentine, coupled with the previous results of the shock‐recovered serpentines, reinforces that serpentine plays a key role to carry water within the snowline of the solar system.