Abstract
Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions. These collisions were expected to occur between kilometre-sized asteroids, generating transient high-pressure states lasting for several seconds to facilitate mineral transformations across the relevant phase boundaries. However, their mechanisms in such a short timescale were never experimentally evaluated and remained speculative. Here, we show a nanosecond transformation mechanism yielding ringwoodite, which is the most typical high-pressure mineral in meteorites. An olivine crystal was shock-compressed by a focused high-power laser pulse, and the transformation was time-resolved by femtosecond diffractometry using an X-ray free electron laser. Our results show the formation of ringwoodite through a faster, diffusionless process, suggesting that ringwoodite can form from collisions between much smaller bodies, such as metre to submetre-sized asteroids, at common relative velocities. Even nominally unshocked meteorites could therefore contain signatures of high-pressure states from past collisions.
Highlights
Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions
Primordial solid materials in the solar system, such as interplanetary dust collected in the Earth’s stratosphere, asteroid regolith recovered by the Hayabusa spacecraft, and chondritic meteorites that have fallen to the Earth, commonly have olivine [α−(Mg,Fe)2SiO4] as their major component mineral[1,2,3,4]
The resulting profile would be more heterogeneous and not consistent with the simulations. This interpretation was supported by time-resolved shock emission signals observed by a streaked optical pyrometer, indicating that a good planar shock of approximately 200 μm in diameter was generated at the ablator/α−Mg2SiO4 interface, where the sample was analysed by X-ray free electron laser (XFEL)
Summary
As seen, the original direction of the oxygen layers was kept in the transformed structure, which would not agree with a transformation occurring by nucleation and growth Another possible rearrangement mechanism from the olivine (α) structure of Mg2SiO4 into the denser polymorphs was theoretically proposed[17], which works without any thermal diffusion of atoms (i.e. it is a diffusionless process), and which directly yields either a ringwoodite (γ) or poirierite (ε) structure. This reflection originally showed zero intensity, owing to the crystallographic reflection conditions for the structure of α (see Methods). This is the most frequently observed high-pressure mineral phase occurring in the shocked meteorites, and that confirms its possible origin in transient compression events occurred in the solid state
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