Numbers, types, and compositions of an unbiased collection of cosmic spherules

Micrometeorites collected from the bottom of the South Pole water well (SPWW) may represent a complete, well-preserved sample of the cosmic dust that accreted on Earth from 1100-1500 A.D. We classified 1588 cosmic spherules in the size range 50-800 mu m. The collection has 41% barred olivine spherules, 17% glass spheres, 12% cryptocrystalline spherules, 11% porphyritic olivine spherules, 12% relic-grain-bearing spherules, 3% scoriaceous spherules, 2% I-type spherules, 1% Ca-Al-Ti-rich (CAT) spherules, and 1% G-type spherules. We also found bubbly glass spherules, spherules with glass caps, and ones with sulfide coatings-particles that are absent from other collections. A classification sequence of the stony spherules (scoriaceous, relic-grain-bearing, porphyritic, barred olivine, cryptocrystalline, glass, and CAT) is consistent with progressive heating and evaporation of Fe from chondritic materials. The modern-day accretion rate and size distribution measured at the SPWW can account for the stony spherules present in deep-sea collection through preferential dissolution of glass and small stony spherules. However, weathering alone cannot account for the high accretion rate of I-type spherules determined for two deep-sea collections. The SPWW collection provides data to constrain models of atmospheric-entry heating and to assess the effects of terrestrial weathering.