Háskóli Íslands

Friday, September 27, 2013 at 15:00, Sturlugata 7, Askja, N-132

Title:  Experimental and field studies of basalt-carbon dioxide interaction

Opponents are Prof. Per Aagaard, Department of Geosciences, University of Oslo, and Prof. Alessandro Aiuppa, Dipartimento di Scienze della Terra e del Mare, Palermo University

Instructors:  Sigurður Reynir Gislason, Domenik Wolff-Boenisch and Eric Oelkers.

The Ph.D. project was supported by European Marie Curie network Delta-Min, SP1-Cooperation CarbFix, the Environmental Fund of Reykjavik Energy, Hitaveita Suðurnesja, Norðurál, University of Iceland, and the Icelandic Centre for Research (RANNÍS).

The defence will be chaired by Magnús Tumi Guðmundsson, Head of the Geology Department

Abstract

The main aim of this study was to design, build, and test a large scale laboratory high pressure column flow reactor (HPCFR) enabling experimental work on water-rock interactions in the presence of dissolved gases, demonstrated here by CO2. The HPCFR allows sampling of a pressurized gas charged fluid along the flow path within a 2.3 m long titanium column filled with mineral, and/or glass particles. In this study, series of experiments were carried out using a carbonated aqueous solution (0.3-1.2 M CO2(aq)) and basaltic glass grains. The scale of the HPCFR, the possibility to sample a reactive fluid at discrete spatial intervals under pressure, and the possibility to monitor the evolution of the dissolved inorganic carbon and pH in-situ all render the HPCFR unique in comparison with other columns constructed for studies of water-rock interactions. Experimental results at ambient temperature showed that the pH of injected pure water evolved from 6.7 to 9-9.5 and most of the dissolved iron was consumed by secondary minerals, similar to natural meteoric water-basalt systems. As CO2-charged water replaced the alkaline fluid within the column, the fluid became supersaturated with respect to carbonates for a short time, but once the entire column was filled with the CO2-charged water and the pH decreased to 4.5, the fluid remained undersaturated with respect to all carbonates. The mobility and concentration of several metals increased significantly in the CO2-fluid phase and some of the metals, including Mn, Fe, Cr, Al, and As exceeded allowable drinking water limits. Iron became mobile and the aqueous Fe2+/Fe3+ ratio increased along the flow path. Basaltic glass dissolution in the CO2-charged water did not overcome the pH buffer capacity of the reactive fluid. The pH rose from an initial pH of 3.4 to 4.5 during the first 40 minutes of CO2-charged water-basaltic glass interaction along the first 18.5 cm of the column but remained constant during the remaining 2.1 meters of the flow path.

In volcanic terrains at high latitude and/or altitude, sub-glacier reservoirs are formed within glaciers by geothermal activity and perhaps small eruptions at the base of ice caps. The reservoirs are periodically drained in glacier floods, called jökulhlaups. Some of these floods, especially those associated with large volcanic eruptions can be disastrous because of their size which is comparable to that of the Amazon River (>200,000 m3/s). In July 2011 two floods  ̴ 2,000 m3/s emerged from Icelandic glaciers (Mýrdalsjökull, Vatnajökull). Sub-glacier reservoirs and the geological basement can be looked upon as a laboratory column flow reactor filled up with rocks of a given chemical composition and surface area, where percolating fluid and external gas source react with each other and with the solid material. The fluid represents melt water and external gas source can represent magmatic gases such as CO2, SO2, HCl and HF.

Water samples collected during both floods had neutral to alkaline pH and conductivity up to 900 µS/cm. Alkalinity present mostly as HCO3- was ~9 meq/kg during the flood peak but stabilized at around 1 meq/kg. Small amount of H2S (up to 1.5 µmol/kg) was detected. Concentrations of most of dissolved constituents including magmatic volatiles Cl-, F- and SO42- in flood water were comparable to the annual concentrations variation of these elements in considered rivers. Comparison of the flood water with Icelandic groundwaters and simple reaction path modelling of fluid chemical evolution suggest that the dissolved element composition of the flood waters developed due to long-time (at least two years) water-rock interaction in presence of limited amount of gases without direct contact of water with magma. This suggests that the origin of the heat source for glacier melting and causing these floods to emerge was geothermal rather than volcanic.

Iwona Galeczka was born 1984 in Tarnowskie Gory, Poland. She received a M.Sc. degree in Hydrogeology and Engineering Geology in 2008 from the University of Science and Technology, Krakow, Poland. Following graduation she worked for one year at the Polish Geological Institute as a junior geologist before moving to Iceland in 2009. 

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