Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments

• Main Author: Tollefsen, Elin
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Stockholms universitet, Institutionen för geologiska vetenskaper 2020
• Subjects: ikaite; petrology; experiment; geochemistry; marine environments; pseudomorphs; calcium carbonate; Ikka Fjord; glendonite; nepheline; siderite; paleotemperature; carbonatite; Geology; Geologi; Greenland; Ikka
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-180194

Carbonates are a group of minerals that play an essential role in several processes on planet Earth, for example in the global carbon cycle and as a product of biomineralisation. Calcite (CaCO3) is by far the most common mineral in the carbonate group, and the stable form of carbonate at Earth surface conditions. However, calcite growth is often kinetically limited and polymorphs of calcite or hydrous calcium carbonates will form instead under certain circumstances. In this thesis, I investigate a hydrous form of calcium carbonate, ikaite (CaCO3 · 6H2O), which occasionally forms under conditions where normally calcite formation would be expected. Ikaite is metastable at surface conditions and has only been observed in nature at temperatures below 7°C. In Ikka Fjord, southwest Greenland, several hundred ikaite columns occur at the bottom of the fjord. Previous studies in Ikka Fjord have shown that ikaite columns are forming above submarine springs that are extremely sodium carbonate rich (pH ~10.5). An association with the surrounding igneous rocks, which comprise nepheline syenite and carbonatite, has been suggested. In the first part of this thesis, I investigate this association. A petrographic study of rocks samples from the igneous complex showed that the combined alteration of the minerals siderite and nepheline could explain the composition of the submarine spring water, and thereby the unique formation of ikaite columns at this site. It is from the mixture of sodium carbonate spring water and seawater that ikaite precipitates in Ikka Fjord, despite the fact that all other calcium carbonates are supersaturated in this mixture. Why ikaite precipitates and not the other forms of calcium carbonate was investigated by a series of experiments in the second and third parts of this thesis. Previous studies have suggested that ikaite was favoured by the low temperature in the fjord (<7°C) and the presence of phosphate (95- 263 μmol/kg) in the submarine spring water, which is known to inhibit calcite growth even ...