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

available online at http://meteoritics.org Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well Susan TAYLOR1*, Gregory F. HERZOG2, and Jeremy S. DELANEY3 1Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire 03755–1290, USA 2Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854–8087, USA 3Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA *Corresponding author. E-mail: Susan.Taylor@erdc.usace.army.mil (Received 18 February 2006; revision accepted 24 November 2006) Abstract–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 × 10−8 g HED micrometeorites/m2-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.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 × 10−8 g HED micrometeorites/m2-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.