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

Abstract

AbstractChemical analysis of groundwater samples collected from a borehole at Hafralækur, 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 Hafralækur. 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.