Abstract

Cavitation is proposed as a mechanism for the discrete generation of high-pressure–high-temperature polymorphs in veins within meteorites and hypervelocity impact structures. Cavitation involves the inception, isothermal growth, and adiabatic implosion of bubbles; their collapse has the potential of generating ultrahigh temperatures and pressures. The formation of pseudotachylytic melt veins within the shocked target is critical to the cavitation process. These veins facilitate bubble growth on decompression and subsequent bubble collapse on relaxation within the liquid. Asymmetric bubble implosion can lead to the rapid inrush of liquid to form a supersonic jet. If the liquid jet strikes a solid surface (i.e., melt-vein walls or suspended clasts) at high velocities, then temperatures of several thousand kelvin and pressures of 10 GPa or more can be realized in the area of jet impact. These conditions are capable of generating high-pressure phases, such as ringwoodite, the spinel-structured polymorph of olivine, which requires formation pressures of >50 GPa. These extreme effects are transient and localized, being restricted to microscopic regions of liquid-solid impact within the cavitating melt vein. The presence of high-pressure polymorphs in meteorites and impact structures does not necessarily imply that the primary shock event attained the conditions necessary for polymorph formation. Instead, the presence of polymorphs may be the result of extreme pressure-temperature excursions due to cavitation.

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