Curious kinetic behavior in silica polymorphs solves seifertite puzzle in shocked meteorite - PubMed (original) (raw)

Curious kinetic behavior in silica polymorphs solves seifertite puzzle in shocked meteorite

Tomoaki Kubo et al. Sci Adv. 2015.

Abstract

The presence of seifertite, one of the high-pressure polymorphs of silica, in achondritic shocked meteorites has been problematic because this phase is thermodynamically stable at more than ~100 GPa, unrealistically high-pressure conditions for the shock events in the early solar system. We conducted in situ x-ray diffraction measurements at high pressure and temperatures, and found that it metastably appears down to ~11 GPa owing to the clear difference in kinetics between the metastable seifertite and stable stishovite formations. The temperature-insensitive but time-sensitive kinetics for the formation of seifertite uniquely constrains that the critical shock duration and size of the impactor on differentiated parental bodies are at least ~0.01 s and ~50 to 100 m, respectively, from the presence of seifertite.

Keywords: high pressure; kinetics; metastable phase; seifertite; shocked meteorite; silica polymorphs.

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Figures

Fig. 1

Fig. 1. Changes of XRD patterns during cold compression followed by heating at high pressures in cristobalite.

(A) Transformations of α-cristobalite (cr) to cristobalite-II (II) and X-I (XI) by compression at room temperature. (B) Formation of seifertite (s) and stishovite (st) by heating at 23 to 25 GPa. au, gold pressure marker; *, graphite sample capsule.

Fig. 2

Fig. 2. Pressure and temperature conditions for the seifertite and stishovite formations from cristobalite.

A datum point above 35 GPa was taken from the previous diamond-anvil cell experiment (10). II, cristobalite-II; XI, cristobalite X-I; s, seifertite; st, stishovite; co, coesite.

Fig. 3

Fig. 3. Kinetics of the seifertite and stishovite formations.

(A) Temperature dependence of the rate constant k. Numbers indicate the pressure condition for each datum point. In the case of the stishovite formation, the pressure dependence was also observed. Activation energies and volumes are summarized in table S2. Results of the fitting are also shown. (B) TTT curves constructed on the basis of kinetic parameters determined in (A).

Fig. 4

Fig. 4. Formation of high-pressure phases from cristobalite and plagioclase (20) on shock event time scales of 1 s (A) and 0.01 s (B).

Metastable phase boundaries of the cristobalite X-I (XI)–to–seifertite (s) transformation and the labradorite (Lb) amorphization are shown in thick solid and dotted lines, respectively. _P_-_T_-t kinetic boundaries for the seifertite and stishovite (st) formations in cristobalite are shown in red and blue lines, respectively. Those for the jadeite (Jd) and stishovite formations in plagioclase (Ab: albite) are shown in dotted black lines.

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