Secondary alteration of the Grønnedal-Ika igneous complex and the genesis of ikaite, CaCO3·6H2O, SW Greenland

Abstract

The mineral ikaite (CaCO3·6H2O) 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 HCO3−. 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 HCO3– 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 CO2 needed to produce the observed mass of tufa columns. We estimated a time frame of ~600 years to produce the necessary CO2 to form the 700 ikaite columns in the fjord.