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(Caco3·6H2O) Formation in Marine Environments

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(Caco3·6H2O) Formation in Marine Environments No.380 Meddelanden från Stockholms universitets institution för geologiska vetenskaper No.380 Elin Tollefsen Experimental, petrological and geochemical investigations of Experimental, petrological and geochemical investigations of ikaite (CaCO investigations geochemical and petrological Experimental, ikaite (CaCO3·6H2O) formation in marine environments Elin Tollefsen 3 ·6H 2 O) formation in marine environments O) formation ISBN 978-91-7911-022-2 Department of Geological Sciences Doctoral Thesis in Geology at Stockholm University, Sweden 2020 Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments Elin Tollefsen Academic dissertation for the Degree of Doctor of Philosophy in Geology at Stockholm University to be publicly defended on Wednesday 13 May 2020 at 10.00 in De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14. Abstract 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 at only trace concentrations. In the second part of this thesis, we simulated Ikka Fjord conditions in laboratory and showed that ikaite precipitation is not controlled by the presence of phosphate in the mixture. Instead, after a second series of experiments I found that it is the presence Mg in seawater that inhibits calcite growth and therefore favour ikaite precipitation. Ikaite is metastable and at temperatures above 7°C the mineral will transform or decompose to calcite and water. The transformation can occur pseudomorphically and pseudomorphs after ikaite have been found worldwide in the sediment record. Pseudomorphs after authigenic ikaite in sediments are named glendonite, and because of the narrow temperature range of ikaite observations in nature, glendonite has been used as a paleotemperature indicator. In the fourth part of this thesis, I explore the temperature range of ikaite nucleation by a series of experiments and found that ikaite nucleation can occur up to at least 35°C. This challenges the use of glendonite as a paleotemperature indicator. Keywords: ikaite, petrology, experiment, geochemistry, marine environments, pseudomorphs, calcium carbonate, Ikka Fjord, glendonite, nepheline, siderite, paleotemperature, carbonatite. Stockholm 2020 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-180194 ISBN 978-91-7911-022-2 ISBN 978-91-7911-023-9 Department of Geological Sciences Stockholm University, 106 91 Stockholm EXPERIMENTAL, PETROLOGICAL AND GEOCHEMICAL INVESTIGATIONS OF IKAITE (CACO3·6H2O) FORMATION IN MARINE ENVIRONMENTS Elin Tollefsen Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments Elin Tollefsen ©Elin Tollefsen, Stockholm University 2020 ISBN print 978-91-7911-022-2 ISBN PDF 978-91-7911-023-9 Printed in Sweden by Universitetsservice US-AB, Stockholm 2020 Till Sofie, Jorunn, Morgane och Edel Abstract 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 circum- stances. 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 ex- pected. Ikaite is metastable at surface conditions and has only been observed in nature at tempera- tures 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 form- ing above submarine springs that are extremely sodium carbonate rich (pH ~10.5). An associ- ation 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 mix- ture. 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 sug- gested 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 at only trace concentrations. In the second part of this thesis, we simulated Ikka Fjord conditions in laboratory and showed that ikaite precipitation is not controlled by the pres- ence of phosphate in the mixture. Instead, after a second series of experiments I found that it is the presence Mg in seawater that inhibits calcite growth and therefore favour ikaite precipita- tion. Ikaite is metastable and at temperatures above 7°C the mineral will transform or decompose to calcite and water. The transformation can occur pseudomorphically and pseudomorphs after ikaite have been found worldwide in the sediment record. Pseudomorphs after authigenic ikaite in sediments are named glendonite, and because of the narrow temperature range of ikaite ob- servations in nature, glendonite has been used as a paleotemperature indicator. In the fourth part of this thesis, I explore the temperature range of ikaite nucleation by a series of experiments and found that ikaite nucleation can occur up to at least 35°C. This challenges the use of glen- donite as a paleotemperature indicator. Sammanfattning Kalciumkarbonater är en grupp mineraler som förekommer inom flera viktiga processer på vår planet, till exempel i den globala kolcykeln och som en produkt inom biomineralisering. Kalcit (CaCO3) är det vanligaste mineralet inom gruppen och det mineral som är stabilt under de förhållanden som råder på jordytan. Kalcitbildningen kan lätt förhindras av kinetiska orsaker, bland annat kan tillväxten stoppas av olika substanser. När detta sker bildas polymorferna aragonit (CaCO3) eller vaterit (CaCO3). Det finns även två vattenhaltiga former av kalciumkarbonat som förekommer naturligt och som också kan bildas i stället för kalcit. Det är monohydrokalcit (CaCO3·H2O) och ikait (CaCO3·6H2O). I den här avhandlingen undersöker jag mineralet ikait (CaCO3·6H2O), och varför det bildas i stället för kalcit i marina miljöer. Ikait är metastabilt på jordytan och mineralet har endast observerats i naturen vid temperaturer mellan -2 till 7°C. På sydöstra Grönland finns flera hundra ikaitkolumner stående på botten i en fjord, Ikkafjorden. Tidigare studier i fjorden har visat att ikaitkolumnerna bildas ovan undervattenskällor som är extremt natrium- och karbonatrika och därför har ett högt pH (~10.5). Fjorden omges av relativt ovanliga magmatiska bergarter, karbonatiter och nefelinsyeniter, som bildades vid kontinental riftning för c. 1.35 miljarder år sedan. Den ovanliga kemin i källvattnet tros bero på omvandlingen av mineraler i dessa bergarter. I den första delen av avhandlingen undersöker jag kopplingen mellan de magmatiska bergarterna och bildandet av ikaitkolumnerna
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