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A shale buchite xenolith with Al-armalcolite and native iron in a lava from Asuk, Disko, central West Greenland

Abstract

A native iron bearing buchite xenolith enclosed in basalt in a native iron bearing lava from Disko is similar in chemistry, apart from volatile components, to unheated Mesozoic to early Tertiary carbonaceous shale sediments from West Greenland, believed to have been the major contaminant and reducing agent causing the formation of the native iron in the volcanic rocks on Disko. The shale buchite has preserved original sedimentary lamination and experienced only a comparatively short-lived heating which allowed only short-range equilibration. The xenolith contains native iron and graphite, while the immediately enclosing basalt does not, and demonstrates the indigenous reducing properties of the shales when heated to high (1,150–1,200 ° C) temperatures at low pressures. Sedimentary clastic iron titanium oxide grains have reacted with the immediately surrounding matrix and completely recrystallized to aggregates of rutile and Al-armalcolite with up to 24 mol% Al2TiO5. The Al-armalcolites contain 5 to 11 mol% of the anosovite (Ti3O5) component. They show a considerable variation in iron-magnesium ratio due to small-scale variations in bulk chemistry to varying f O2 and to the effects of progressive melting of the sediment. Glassy veins in the buchite, formed along original fractures in the sediment, contain an assemblage of cordierite, low Ca-pyroxene, plagioclase, ilmenite, ferropseudobrookite-rich armalcolite, iron metal, troilite and acid glass, and crystallized under higher oxygen fugacities than the buchite interior, probably due to influx from the less reduced enclosing basalt magma.

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Pedersen, A.K. A shale buchite xenolith with Al-armalcolite and native iron in a lava from Asuk, Disko, central West Greenland. Contr. Mineral. and Petrol. 69, 83–94 (1979). https://doi.org/10.1007/BF00375196

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Keywords

  • Shale
  • Rutile
  • Ilmenite
  • Cordierite
  • Carbonaceous Shale