• Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well

      Taylor, Susan; Herzog, Gregory F.; Delaney, Jeremy S. (The Meteoritical Society, 2007-01-01)
      Ten glass cosmic spherules (CS) from the South Pole water well collection were analyzed by electron microprobe. Nine of them have Fe/Mn and Fe/Mg ratios in the range typical of chondrites. One of them (SP37-3), along with up to six other previously analyzed CS, have nonchondritic Fe/Mn and Fe/Mg ratios that agree well with values typical of either (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not of lunar samples. SP37-3 also contains an anorthite relic grain. Anorthite has not previously been reported in cosmic spherules, but is well known in HED meteorites. The much greater frequency of HEDs among hand-sized meteorites suggests but does not prove that HED precursors are more likely for the nonchondritic spherules.We estimate that HED-like micrometeorites constitute ~0.5 +/- 0.4% of the total population of micrometeorites in the South Pole water well, a fraction that translates to a flux of 1.6 +/- 0.3 x 10^(-8) g HED micrometeorites/m^2-y. The ratio of HED-like objects to carbonaceous objects is about 100 times less in micrometeorites than among hand-size specimens. We infer that the comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation.
    • Geochemistry of 4 Vesta based on HED meteorites: Prospective study for interpretation of gamma ray and neutron spectra for the Dawn mission

      Usui, Tomohiro; McSween, Harry Y. (The Meteoritical Society, 2007-01-01)
      Asteroid 4 Vesta, believed to be the parent body of the howardite, eucrite, and diogenite (HED) meteorites, will be investigated by the Dawn orbiting spacecraft. Dawn carries a gamma ray and neutron detector (GRaND) that will measure and map some major- and trace-element abundances. Drawing on HED geochemistry, we propose a mixing model that uses element ratios appropriate for the interpretation of GRaND data.Because the spatial resolution of GRaND is relatively coarse, the analyzed chemical compositions on the surface of Vesta will likely reflect mixing of three endmember components: diogenite, cumulate eucrite, and basaltic eucrite. Reliability of the mixing model is statistically investigated based on published whole-rock data for HED meteorites. We demonstrate that the mixing model can accurately estimate the abundances of all the GRaND-analyzed major elements, as well as of minor elements (Na, Cr, and Mn) not analyzed by this instrument. We also show how a similar mixing model can determine the modal abundance of olivine, and we compare estimated and normative olivine data for olivine-bearing diogenites. By linking the compositions of well-analyzed HED meteorites with elemental mapping data from GRaND, this study may help constrain the geological context for HED meteorites and provide new insight into the magmatic evolution of Vesta.