Secondary alteration of the Grønnedal-Ika igneous complex and the genesis of ikaite, CaCO 3 ·6H 2 O, SW Greenland

Elin Tollefsen*, Gabrielle Stockmann, Alasdair Skelton, Lena Lundqvist, Erik Sturkell

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The mineral ikaite (CaCO 3 ·6H 2 O) precipitates from a mixture of spring water and seawater as tufa columns which grow at a rate of up to 50 cm per year reaching heights of up to 18 m in Ikka Fjord, SW Greenland. In the fjord, column formation occurs only at the base of a nepheline syenite‑carbonatite complex that flanks the fjord and an association has therefore been proposed. The spring water that seeps up at the bottom of the fjord is oversaturated in Na + and HCO 3 . In this study, we show that these ions were acquired by alteration reactions in the syenite‑carbonatite complex: Na + is released during replacement of nepheline by illite and analcime in nepheline-syenite rocks and HCO 3 is released by oxidation of siderite to goethite in carbonatite rocks. The chemically charged groundwater mixes with seawater and gives rise to the formation of the tufa columns. We performed a mass balance to show that the mass of the carbonatite in the complex is more than sufficient to provide the CO 2 needed to produce the observed mass of tufa columns. We estimated a time frame of ~600 years to produce the necessary CO 2 to form the 700 ikaite columns in the fjord.

Original languageEnglish
Pages (from-to)18-30
Number of pages13
JournalChemical Geology
Volume510
DOIs
Publication statusPublished - 5 Apr 2019

Bibliographical note

Publisher Copyright:
© 2019

Other keywords

  • Alteration
  • Carbonatite
  • Ikaite
  • Nepheline
  • Nepheline syenite
  • Pseudomorphs
  • Siderite

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