科研之友
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摘要
Nine howardites and two diogenites were recovered from the Pecora Escarpment Icefield (PCA) in 2002. Cosmogenic radionuclide abundances indicate that the samples are paired and that they constituted an approximately 1 m (diameter) meteoroid prior to atmospheric entry. At about 1 m in diameter, the PCA 02 HED group represents one of the largest single pre-atmospheric pieces of the Vestan surface yet described. Mineral and textural variations were measured in six of the PCA 02 howardites to investigate meter-scale diversity of the Vestan surface. Mineral compositions span the range of known eucrite and diogenite compositions. Additional non-diogenitic groups of Mg- and Fe-rich olivine are observed, and are interpreted to have been formed by exogenic contamination and impact melting, respectively. These howardites contain olivine-rich impact melts that likely formed from dunite- and harzburgite-rich target rocks. Containing the first recognized olivine-rich HED impact melts, these samples provide meteoritic evidence that olivine-rich lithologies have been exposed on the surface of Vesta. Finally, we present a new method for mapping distributions of lithologies in howardites using 8 elemental X-ray maps. Proportions of diogenite and eucrite vary considerably among the PCA 02 howardites, suggesting they originated from a heterogeneous portion of the Vestan surface. While whole sample modes are dominated by diogenite, the finer grain size fractions are consistently more eucritic. This discrepancy has implications for near-infrared spectral observations of portions of Vestas surface that are similar to the PCA 02 howardites, as the finer grained eucritic material will disproportionately dominate the spectra.
- 出版日期2012-6
