Gabrielle Stockmann

Dissolution of diopside and basaltic glass: the effect of carbonate coating

Abstract Far-from-equilibrium dissolution experiments with diopside and basaltic glass in mixed-flow reactors at 70°C and pH 8.2 show that solute concentrations do not reach steady state over the experimental duration of 45-60 days. Chemical modelling indicates that during the dissolution experiments, solutions have become supersaturated with respect to carbonates in the case of diopside, and carbonates, clay minerals and zeolites in the case of the basaltic glass. Decreasing dissolution is therefore interpreted as a result of secondary surface precipitates blocking the reactive surface area. Calcite formation was supported in both experiments by a significant increase in Ca (and Sr) concentrations as pH was abruptly lowered from 8.2 to 7 because this change increased carbonate solubility and caused all potential carbonate precipitates to re-dissolve. The reduction in pH also led to an increase in Si concentration for diopside and a decrease in Si concentration for basaltic glass. This observation is in accordance with previous experiments on the pH-dependent dissolution rates of pyroxenes and basaltic glass.

Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake

Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15–20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8–10 years after the earthquake.

Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland

Chemical analysis of groundwater samples collected from a borehole at Hafralaekur, northern Iceland, from October 2008 to June 2015 revealed (1) a long-term decrease in concentration of Si and Na and (2) an abrupt increase in concentration of Na before each of two consecutive M > 5 earthquakes which occurred in 2012 and 2013, both 76 km from Hafralaekur. Based on a geochemical (major elements and stable isotopes), petrological, and mineralogical study of drill cuttings taken from an adjacent borehole, we are able to show that (1) the long-term decrease in concentration of Si and Na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and (2) the abrupt increase of Na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of Na into groundwater. We attribute decay of the Na peaks, which followed and coincided with each earthquake to uptake of Na along fractured or porous boundaries between labradorite and analcime crystals. Possible causes of these Na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components. Both could have been triggered by preseismic dilation, which was also inferred in a previous study by Skelton et al. (2014). The mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown.