Are there signs of a large Paleocene impact, preserved around Disko Bay, West Greenland? Nuussuaq spherule beds origin by impact instead of volcanic eruption?
Published:January 01, 2005
A.P. Jones, A.T. Kearsley, C.R.L. Friend, E. Robin, A. Beard, A. Tamura, S. Trickett, P. Claeys, 2005. "Are there signs of a large Paleocene impact, preserved around Disko Bay, West Greenland? Nuussuaq spherule beds origin by impact instead of volcanic eruption?", Large Meteorite Impacts III, Thomas Kenkmann, Friedrich Hörz, Alex Deutsch
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On the Nuussuaq peninsula, Western Greenland sedimentary deposits of glass spherules also contain high Ir, Co, Ni, and Cu anomalies. The iron-rich silicate glass spherules (to ∼3 wt% NiO, ∼35 wt% FeO) are highly circular in cross section. They show surface dissolution, smectite replacement and calcite infilling of vesicles, though many glasses are optically unaltered. They are strikingly heterogeneous, with schlieren outlining counter-flowing convection cells. Their pronounced Fe-Ni correlation is unlike volcanic suites, but is explained by mixing between basaltic melt and an enriched iron-nickel source. Distinctive nickel-spinel (∼7–10 wt% NiO) contains very nickel-rich cores. Occasional glass spherules show compositional gradients toward resorbed silicates, (plagioclase, clinopyroxene); isotropic plagioclase has anomalous texture comparable to impact-melted lunar breccias. Their anomalously high copper and sulfur (to ∼1%) have lead to an explanation as products of fire-fountaining of exotic or picritic Disko lavas; they would be perhaps the only non-impact occurrence of Ni spinel. Since their discovery, better criteria for recognition of spherules ejected from large impacts have been established, and greater variations in meteorite chemistry as potential projectiles have been described. New mineralogical and petrographic textural data for the Nuussuaq spherules suggest they should be reinterpreted as impact ejecta; the highly oxidized Ni-spinel is a very characteristic signature of meteorite impact ejecta. Delicate preservation features rule out substantial sedimentary reworking, and spherule bed thicknesses imply a large source crater. Stratigraphically, the spherule beds are poorly constrained, but nannofossils and magnetostratigraphy place them close to onset of the West Greenland flood lavas (ca. 61–62 Ma). They share many characteristics with massive native iron localities in dykes and lavas up to >100 km away on Disko (Qeqertarsuaq) Island, but their precise relationship remains to be established.