High-pressure and high-temperature experiments on albitic feldspar have shown that albite dissociates into coesite + silica phases (quartz, coesite and stishovite) under high-pressure and -temperature conditions (Fig. 3 and Refs. 16,17,18,19,20,21,22). Impact-induced formation of jadeite from feldspar has been reported in shocked terrestrial rocks and meteorites23,24,25,26,27,28,29. Previously reported jadeite has granular or stringer-like morphologies and no significant compositional difference exists between albitic feldspar in host-rock and jadeite-bearing grains in shock-melt veins28,29. Detailed transmission electron microscope (TEM) observations on natural and synthetic jadeite formed from feldspar indicated that the previously studied natural jadeite was formed from crystalline or amorphized albitic feldspar through a solid-state reaction29. On the other hand, jadeite in Chelyabinsk meteorite has needle-like or skeletal-rhombic morphologies, which suggest rapid crystallization of jadeite. The compositional difference between albitic feldspar in host-rock and the bulk jadeite-bearing grains in the shock-melt veins suggests that the original albitic feldspar entrained in the shock-melt veins experienced melting, in which elemental migration of K occurred (Supplementary Table S1). The compositional difference between jadeite-bearing parts and coexisting jadeite-free feldspathic glass parts also suggests long-distance atomic diffusion of Na and K during jadeite formation. Such elemental migrations seem to be difficult in solid-state reaction during the short duration of an impact. These features suggest that jadeite in Chelyabinsk meteorite was formed by rapid crystallization from molten albitic feldspar.

Figure 3 Pressure-temperature phase diagram of albite and olivine. The blue lines represent phase boundaries for albite16,17,18,19,20,21,22, whereas the green lines represent those for olivine (Fo 71 ) (ref. 31). A possible liquidus curve of the host-rock (LL chondrite) is also shown by the gray dashed line33,34. The hatched area in red color indicates the estimated pressure-temperature conditions for coarse-grained fragments in the shock-melt veins during the impact. Ab = albite, Jd = jadeite, Coe = coesite, Sti = stishovite, Lgn = lingunite (NaAlSi 3 O 8 with a hollandite structure), L = Albite liquid, Ol = olivine, Rwd = ringwoodite, Wds = wadsleyite. Full size image

In the solid-state feldspar dissociation reaction or slow crystallization of albitic melt, some silica phases (quartz, coesite or stishovite) should coexist with jadeite. However, any silica phases were not identified with jadeite in Chelyabinsk meteorite. The same thing has also been reported for other impact-induced jadeite in terrestrial rocks and meteorites23,24,25,26,27,28,29. The crystallization kinetics of jadeite and silica phases from amorphized albitic feldspar was studied by Kubo et al.30. Their results suggest that nucleation of silica phases is considerably delayed compared with that of jadeite. The absence of silica phases in jadeite-bearing grains can be due to short duration of high-pressure and -temperature conditions during an impact event.

Figure 3 shows a pressure-temperature phase diagram of albite and olivine (Fo 71 ) (Refs. 16,17,18,19,20,21,22, 31). Albite dissociates into jadeite + quartz at about 3 GPa and the silica phase changes to coesite or stishovite with increasing pressure. Above 19 GPa, lingunite (NaAlSi 3 O 8 with a hollandite structure) or an assemblage of NaAlSiO 4 with a calcium ferrite structure + stishovite can be formed. Thus, jadeite can be stable at 3–19 GPa as a liquidus or subsolidus phase. Olivine (Fo 71 Fa 29 ) transforms to its high-pressure polymorphs (ringwoodite or wadsleyite) from 9–12 GPa31. Enstatite (En 75 Fs 25 Wo 0 ) transforms to majorite at 17 GPa and 1800°C32. In this study, we identified jadeite in the shock-melt veins of Chelyabinsk meteorite, whereas any high-pressure polymorphs of olivine or pyroxene have not been identified so far. It suggests that the equilibrium shock pressure of the jadeite-forming impact was at least 3–12 GPa.

The shock-melt vein matrix is considered to have crystallized from a melt with the bulk Chelyabinsk meteorite composition. Thus, temperature of the shock-melt vein matrix would have been higher than the liquidus temperature of bulk LL chondrite. The liquidus temperature of bulk LL chondrite can be estimated as intermediate between those of Allende carbonaceous chondrite and KLB1 peridotite33,34. When we assume the pressure condition to be 3–12 GPa, a liquidus temperature of LL chondrite can be estimated to be 1700–2000°C (Fig. 3). Therefore, pressure-temperature conditions of the shock-melt vein matrix are estimated to be 3–12 GPa and over 1700–2000°C. For coarse-grained fragments, which are not completely molten, the conditions would be 3–12 GPa and below 1700–2000°C (Fig. 3).

Shock-melt veins cools down and solidifies by conduction of heat to surrounding relatively cool host-rock. Cooling and solidification of a melt vein starts from the interface between host-rock and veins and finishes at the center of the veins. If jadeite exists at the center of a shock-melt vein, it may indicate that the shock-melt vein completely solidified under pressure condition of 3–12 GPa. The maximum width of shock-melt veins which contain jadeite at the center is about 1 mm. When we assume temperature of the host-rock and shock-melt vein during the shock compression as 100°C and 2000°C respectively, the solidification time of the shock-melt vein is calculated to be ~70 ms based on the cooling speed analysis of shock-melt veins by Langenhorst and Poirier7,35 (Details of the calculations are provided in Supplementary information). When the vein completely solidifies, the temperature within the vein is still high (>1100°C). If pressure release occurs at this time, jadeite might vitrify or back-transform to low-pressure phases due to the ambient pressure and high-temperature conditions. Thus, the shock pressure duration could be longer than 70 ms.

Using the Rankine-Hugoniot's relations, we can calculate the size and impact velocity of the impactor which caused the jadeite-forming impact8,36,37. The estimated shock pressure of 3–12 GPa and its duration of >70 ms correspond to a scenario that an impactor larger than 0.15–0.19 km collided with a parent body of Chelyabinsk meteorite with a relative speed of 0.4–1.5 km/s (Details of the calculations are provided in Supplementary information). The impact velocity seems to be consistent with those of impacts in the main asteroid belt38. Popova et al. suggested that the Chelyabinsk parent body experienced a significant thermal and/or collision event 4,452 ± 21 Ma ago based on the U–Pb radiometric dating of apatite3. On the other hand, Galimov et al. suggested that Chelyabinsk meteorite records an impact event at 290 Ma ago based on Sm–Nd isotopic systematics of whole rock samples6. The impact event which formed the pervasive shock-melt veins and jadeite was probably the last intense impact event recorded in Chelyabinsk meteorite. If there were subsequent thermal or more intense impact processes, jadeite could not have survived. Thus, the impact event studied here might have occurred at or after 290 Ma ago and the Chelyabinsk asteroid probably separated from its parent body at this event. Dynamical lifetimes of asteroids placed on the orbital resonances of the main asteroid belt were estimated to be less than 10 Ma39. The Chelyabinsk asteroid blasted off from its parent body could have moved into an orbital resonance at <10 Ma ago and then been delivered into an Earth-crossing orbit.