Fractionation and fragmentation of glass cosmic spherules during atmospheric entry

Abstract

Glass cosmic spherules are a sub-type of S-type cosmic spherules that are almost at the end phase of fractionation and volatilization during atmospheric entry. A focused investigation of 36 glass spherules out of several hundred spherules reveals fractionation and fragmentation phenomena. During atmospheric entry, some of the glass spherules largely preserve relict pyroxene normative crystals within. The spherules appear to have experienced temperatures of ∼1700°C that resulted in vaporization of Fe and other volatile elements such as S, Na, and K from the silicate spherule, resulting in the enrichment of SiO2, MgO, and CaO in weight percent. Calculations with a chemical ablation model suggest an entry velocity of ∼11–16km/s to achieve the temperature required for glass spherule formation under these conditions the micrometeoroid undergoes a mass of loss of ∼20%. Any increase in entry velocity will increase the mass loss drastically and change its chemical composition. Most glass spherules beyond a certain threshold diameter of ∼300μm appear to undergo fragmentation resulting in two different spherules, which could lead to an apparent enhancement of the percentage of glass spherules in unbiased collections. Siderophile and volatile element loss and subsequent development of a platinum group element (PGE) nugget are observed in one of the spherules. This appears to be the first reported observation of a platinum group nugget (PGN) in a glass spherule. The nugget has a diameter of ∼3μm and contains all the six PGE, although some of the more volatile PGE are depleted relative to an average chondritic composition. Interestingly, the PGE data of the nugget seem to have a trend which is similar to that of a PGE nugget from a calcium aluminium rich inclusion (CAI). The trace element data of these glass spherules suggest that their parent bodies were carbonaceous chondrites.