Sigurdur R. Gislason

The mechanism, rates and consequences of basaltic glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25°C and pH = 3 and 11

Steady state basaltic glass dissolution rates were measured as a function of aqueous aluminum, silica, and oxalic acid concentration at 25° C and pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basaltic glass, and exhibited stoichiometric Si versus Al release. Rates are independent of aqueous silica activity, but decrease with increasing aqueous aluminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experimental uncertainty using where r signifies the surface area normalized basaltic glass steady state dissolution rate, k refers to a rate constant equal to 10-11.65 (mol of Si)/cm2/s, and ai represents the activity of the subscripted aqueous species. The observation that all rates obtained in the present study can be described by a single regression equation supports strongly the likelihood that basaltic glass dissolution is controlled by a single mechanism at both acidic and basic pH and in both the presence and absence of organic acids. Taking account of the dissolution mechanisms of similarly structured and compositioned minerals, and previously published studies of basaltic glass dissolution behavior, basaltic glass dissolution likely proceeds via 1) the relatively rapid and essentially complete removal of univalent and divalent cations from the near surface; 2) aluminum releasing exchange reactions between three aqueous H+ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. The breaking of Al-O bonds does not destroy the glass framework; it only partially liberates the silica tetrahedral chains by removing adjoining Al atoms. Basaltic glass dissolution rates are proportional to the concentration of partially detached framework Si tetrahedra near the surface, which is linked through the law of mass action for the Al/proton exchange reaction to aqueous aluminum activity. Copyright

Mechanism, rates, and consequences of basaltic glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a function of pH and temperature

This study is aimed at quantifying surface reaction controlled basaltic glass dissolution rates at far-from-equilibrium conditions. Towards this aim, steady-state basaltic glass dissolution rates were measured as a function of pH from 2 to 11 at temperatures from 6° to 50°C, and at near neutral conditions to 150°C. All rates were measured in open system titanium mixed flow reactors. Measured dissolution rates display a common pH variation; dissolution rates decrease dramatically with increasing pH at acid conditions, minimize at near neutral pH, and increase more slowly with increasing pH at basic conditions. The pH at which basaltic glass dissolution minimizes decreases with increasing temperature. Dissolution rates were interpreted within the context of a multioxide dissolution model. Constant temper- ature rates are shown to be consistent with their control by partially detached Si tetrehedra at the basaltic glass surface. Regression of far-from-equilibrium dissolution rates obtained in the present study and reported in the literature indicate that all data over the temperature and pH range 6° T 300°C and 1 pH 11 can be described within uncertainty using

Meteoric water-basalt interactions. I: A laboratory study

Rates, stoichiometry and activation energy of dissolution, pH vs. time and activity-activity paths were determined by dissolving basaltic rocks under simulated natural conditions at 25 to 60°C. Dissolution follows a linear rate law, with basaltic glass dissolving about 10 times faster than crystalline basalt. Rates are independent of pH from 7 to 9.5. The basaltic glass dissolves near stoichiometrically at 25°C, but this is not so for the crystalline basalt. The average activation energy for dissolution of basaltic glass is 32 kJ/ mol (±3). For individual elements leached from crystalline basalt, it ranges from 35 to 15 kj/mol. This indicates that under experimental conditions reactions on the surfaces of the solids are the rate-determining step in the dissolution mechanism. Ca, Mg, and Si reached steady state or equilibrium concentrations in solutions at 45 and 65°C. Alteration minerals formed in vugs on the solids but did not form a continuous protective layer on the solids. Reaction paths for the experimental solutions are strongly dependent on their pH versus time evolution. Solutions sealed off from the atmosphere evolve into the Ca-zeolite field, whereas those open to the atmosphere evolve into the clay mineral fields. During the experiments the consumption of hydrogen by solids at 25°C (10−5 mol/kg) is not sufficient to significantly change the hydrogen isotope ratio of the experimental solutions.

Fertilizing potential of volcanic ash in ocean surface water

The fertilization potential of newly erupted and well-preserved ash from the 2000 Hekla eruption in Iceland was measured for the first time by flow-through experiments. As previously shown, (1) the North Atlantic Ocean, including the subarctic seas surrounding Iceland, is the largest net sink of the world’s oceans for atmospheric CO 2, owing to biological drawdown during summer; (2) almost complete consumption of phosphate in chlorophyll-rich areas of the North Atlantic Ocean might limit primary production; and (3) in the southern Pacific Ocean and parts of the equatorial Pacific Ocean iron might limit primary production. We found through laboratory experiments that volcanic ash exposed to seawater initially releases large amounts of adsorbed phosphate, 1.7 mmol·g 21 ·h 21 ; iron, 37.0 mmol·g 21 ·h 21 ; silica, 49.5 mmol·g 21 h 21 ; and manganese, 1.7 mmol·g 21 ·h 21 . Dissolution of acid aerosols adsorbed to the surface of the ash caused the high initial release of major and trace elements. Because of the instantaneous dissolution of adsorbed components when newly erupted volcanic ash comes in contact with the ocean surface water, macronutrients and ‘‘bioactive’’ trace metals are released fast enough to become available to support primary production.

Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment

On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions

Inject, baby, inject! Atmospheric CO2 can be sequestered by injecting it into basaltic rocks, providing a potentially valuable way to undo some of the damage done by fossil fuel burning. Matter et al. injected CO2 into wells in Iceland that pass through basaltic lavas and hyaloclastites at depths between 400 and 800 m. Most of the injected CO2 was mineralized in less than 2 years. Carbonate minerals are stable, so this approach should avoid the risk of carbon leakage. Science, this issue p. 1312 Basaltic rocks may be effective sinks for storing carbon dioxide removed from the atmosphere. Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO2. This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO2 emissions through mineralization can be far faster than previously postulated.

The 1991 eruption of Hekla, Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s−1. After this peak, the effusion rate declined rapidly to 10–20 m3 s−1, then more slowly to 1 m3 s−1, and remained at 1–12 m3 s−1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.

Meteoric water-basalt interactions. II. A field study in N. E. Iceland

The compositions of rain, snow, melt, spring and geothermal waters from the rift zone of N.E. Iceland can be explained by seaspray addition, chemical fractionation at the seawater-air interface, burning of fossil fuel, farming activities, purification by partial melting of snow and ice, dissolution of basalts and buffering by alteration minerals. The dissolution of the rocks appears to be incongruent. During solute acquisition, spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields. All but four of the springs are undersaturated with respect to calcite. Silica concentrations are compatible with the solubility of basaltic glass. The reactions reflected in the spring waters appear to have taken place sealed off from atmospheric CO2 after initial saturation. The geothermal waters which are recharged by the spring waters are depleted in Mg and Ca but enriched in carbon and sulfur with respect to dissolution of primary rocks. Expressions are derived relating dissolution rates of rocks, age of groundwaters, physical properties of groundwaters and mass transfer. The characteristic rock particle radii in the cold water aquifers range from 0.2 to 2 cm and the characteristic crack openings are of the order 0.04 to 0.4 cm. Using laboratory studies on the Icelandic lavas as a guide, the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years. The average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0.6 to 6 m2 and these 1000 g of water have reacted with 0.1 to 1 g of basaltic rocks. The same mass of thermal water has interacted with 100 to 300 g of unaltered rocks.

Dissolution of primary basaltic minerals in natural waters: saturation state and kinetics

Abstract The state of saturation of olivine, orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in Iceland including river waters, groundwater and geothermal waters with temperatures up to 250°C. The stability of olivine and orthopyroxene decreases with increasing Mg content. Similarly, the stability of plagioclase decreases with increasing anorthite content. The river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene, the degree of undersaturation being dominated by the water pH. All river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the pH is 200°C) are undersaturated with olivine, orthopyroxene and plagioclase, the first two particularly when Fe rich, and sufficient to cause olivine to dissolve according to the linear rate law. At intermediate temperatures (50–150°C) the geothermal waters are close to equilibrium with these minerals except for Mg-rich olivine. It seems likely that dissolution of glass from basalt, which is very reactive, will favor stability of the igneous minerals.

Seafloor weathering controls on atmospheric CO2 and global climate

Alteration of surficial marine basalts at low temperatures (<40°C) is a potentially important sink for atmospheric CO2 over geologic time. Petrologic analyses, thermodynamic calculations, and experimental weathering results point to extensive Ca leaching and consumption of marine CO2 during alteration. Basalt weathering in seawater-like solutions is sensitive to temperature. The activation energy for initial basalt weathering in seawater is 41–65 kJ mol−1. If seafloor weathering temperatures are set by deep ocean fluids under high fluid to rock ratios the feedback between weathering and atmospheric CO2 is indirect, but sizeable. If the bulk of seafloor weathering occurs in the presence of low-temperature hydrothermal fluids, the weathering feedback depends on the linkage between spreading rates and heat flow. In either case, the primary linkage between seafloor weathering and the global carbon cycle appears to be thermal as opposed to chemical.