open access

https://hdl.handle.net/2115/75759

Survivability of presolar oxygen isotopic signature of amorphous silicate dust in the protosolar disk

英語

2019-08-12

John Wiley & Sons

Meteoritics & Planetary Science

55(6)

1281-1292

Oxygen isotope exchange experiments between tens of nanometer-sized amorphous enstatite grains and water vapor were carried out under a condition of protoplanetary disk-like low water vapor pressure in order to investigate the survivability of distinct oxygen isotope signatures of presolar silicate grains in the protosolar disk. Oxygen isotope exchange between amorphous enstatite and water vapor proceeded at 923-1003 K and 0.3 Pa of water vapor through diffusive isotope exchange in the amorphous structure. The rate of diffusive isotope exchange is given by D (m(2) s(-1)) = (5.0 +/- 0.2) x 10(-21) exp[-161.3 +/- 1.7 (kJ mol(-1)) R-1 (1/T-1/1200)]. The activation energy for the diffusive isotope exchange for amorphous enstatite is the same as that for amorphous forsterite within the analytical uncertainties, but the isotope exchange rate is similar to 30 times slower in amorphous enstatite because of the difference in frequency factor of the reaction. The reaction kinetics indicates that 0.1-1 mu m-sized presolar amorphous silicate dust with enstatite and forsterite compositions would avoid oxygen isotope exchange with protosolar disk water vapor only if they were kept at temperatures below similar to 500-650 K within the lifetime of the disk gas.

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