CHARACTERISTICS AND ORIGIN OF THE GLASS SPHERULES FROM THE PALEOCENE FLOOD-BASALT PROVINCE OF WESTERN GREENLAND

Glass spherules have been found in several Paleocene sandstone beds in central Nuussuaq, western Greenland. These beds are a mixture of glas s, smectite, and calcite with rare occurrence of graphite and metal. T he glass varies from a basaltic to a basic-andesitic composition and d isplays, on a micrometer scale, Mg, Al, Ca, Fe, Ni, and Cu enrichments relative to the bulk composition. These enrichments are associated wi th resorbed silicate crystals (plagioclase, pyroxene, and olivine) and partly oxidized metal-sulphide inclusions. The spherules have high Co , Ni, Cu, and Ir concentrations positively correlated with Fe. The hig h Cu content of the glass and the nonmeteoritic Ir/Cu and Ir/Ni (or Ir /Co) ratios of the spherules exclude a meteoritic origin for these ele ments. These compositional characteristics indicate a purely terrestri al origin, and likely result from the contamination of a basaltic-ande sitic melt by sulphide liquid formed through reduction of a magma by C -rich sediments. This mechanism has already been proposed to account f or the occurrence of Fe-bearing basalts and andesites in the Nuussuaq and Disko area. This origin is further supported by the high Cr conten t of the glasses, consistent with a highly magnesian tholeiitic picrit e basalt for the parent magma, and the presence of metal, sulphide, an d graphite in the spherule layers. The occurrence in the glass of roun ded magnetite resulting from the oxidation of sulphide, and of dendrit ic spinel crystals characteristic of a rapid growth from a high temper ature melt, show that the spherules were quenched in an oxidizing envi ronment. We conclude that Nuussuaq glass spherules result from rapid a nd incomplete oxidation of liquid droplets generated by one or several episods of lava fountains.