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

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.