Ultrafast olivine-ringwoodite transformation during shock compression

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.

Fig. 1. Experimental system.

The α − Mg₂SiO₄ single crystal was shock-compressed along its crystallographic a-axis by irradiation of the power laser pulse into an ablator of polypropylene film, where the crystal structure was simultaneously analysed using the XFEL pulse. A thin polycrystalline Al₂O₃ plate (not shown) was fixed on the upper surface of α − Mg₂SiO₄ when the shock arrival time was evaluated.

Fig. 2. XFEL diffraction results showing time evolution of g = 200 reflections from the polypropylene-olivine-Al₂O₃ layered target.

a Two dimensional images where each number represents the delay time t of an XFEL pulse from the power laser pulse arrival time to the surface of polypropylene. Diffuse spots marked ‘E’, ‘P’, and ‘D’ show the reflection from the oxygen layers of olivine in elastic and plastic shock wave regions, and reflection from its denser recrystallised region, respectively. Crystallographic orientations (a and b) are approximately shown for readability, together with the direction of a-axis compression. These axes are not necessarily precisely located on the detector plane. A vertical linear profile within the three orange strips with the Laue indices shows Bragg reflections from the (104), (110), and (113) planes of polycrystalline Al₂O₃. b One dimensional patterns as a function of d obtained by the integration of the images. Positions of the peaks marked with E, P and D show d of their corresponding reflections, d₂₀₀^E, d₂₀₀^P, and d₂₀₀^D, respectively. At t = 4 ns, the d₂₀₀^P was compressed to 94 ± 1 % of d₂₀₀^1atm, which was comparable to the a-axis compression at static pressure of 60 to 100 GPa^,. At t = 6 ns, the E and P peaks become much stronger due to the increase in volume of these compressed regions (the shown intensity profile was reduced by a factor of 4). At t = 7 ns, the ‘E’ and ‘P’ peaks moved further to the left to become almost undetectable (see ‘Methods’), while the ‘D’ peak started to grow (d₂₀₀^D = 2.25−2.26 Å, corresponding to d₂₂₂ of ringwoodite at 21 to 25 GPa). At t = 8 ns, the newly-emerged ‘D’ peak rapidly grew; it was also gradually shifting to the right (d₂₀₀^D = 2.25 to 2.31 Å, corresponding to d₂₂₂ of ringwoodite at 6 to 25 GPa). At t = 9 ns, the reflections of Al₂O₃ suddenly became slightly compressed, indicating that the shock wave had travelled throughout the olivine plate to arrive there. The strips with orange (not compressed) and red (compressed) colours are visual guides for the behaviour of these Al₂O₃ peaks.

Fig. 3. XFEL diffraction images showing time evolution of g = 300 reflection from the polypropylene-olivine layered target.

Crystallographic orientations (a, b, and c) are approximately shown for readability together with the direction for a-axis compression. These axes are not necessarily exactly located on the detector plane. White squares show the position of g = 300, and their magnified views were separately shown at the right of the images. a Series images along the b direction, indicating that the g = 300 reflection was not deformed. b Series images along the c direction, indicating that the g = 300 reflection was extensively deformed. Laue indices of the other reflections together with the images of wider area coverage are shown in Supplementary Fig. 3.

Fig. 4. Schematic illustrations of shearing mechanism of Mg₂SiO₄.

a Perspective views of oxygen anion layers, showing how the original α structure (1 atm) was compressed along its a-axis (P), and then recrystallised into the dense structure (D) induced by fast shearing (slipping) motions of these layers at the slip planes shown with brown colour. The original α structure consists of alternating A and B layers of oxygen forming a hexagonal close packing (hcp) arrangement. The dense polymorph structure (β, γ, or ε) all consists of alternating A, B, and C layers of oxygen forming a cubic close packing (ccp) arrangement. b The shearing model toward the γ structure, where cooperative slip motions of the oxygen layers occur along the c axis of the α structure, [001]_α. Only the oxygen anions are shown for clarity. See Supplementary Fig. 4 for the accompanying cooperative motions of magnesium and silicon cations. The unit cell of α structure is indicated by the rectangle with dotted lines. The upper layer B slips with respect to the lower layer A by partial dislocation with Burgers vector b_p = 1/12[013]_α, as indicated by the thick solid pink arrow. The other partial dislocations were indicated by the thin solid pink arrow, where all these partial dislocations in total constitute the perfect dislocation with the Burgers vector b_t = [001]_α that defines the direction of macroscopic shearing along the c axis^,. c The shearing model toward the ε structure, where cooperative slip motions of the oxygen layers occur along the b axis of the α structure, [010]_α. The unit cell of α structure is shown by the rectangle with dotted lines. The layer B slips with respect to the layer A by partial dislocation with b_p = 1/3[010]_α, as indicated by the thick solid pink arrow. The other partial dislocations were indicated by the thin solid pink arrow, where all these partial dislocations in total constitute the perfect dislocation with b_t = [010]_α that defines the direction of macroscopic shearing along the b axis.