Coupling Between Mineral Reactions and Chemical Changes in Groundwater Before and After Earthquakes in Iceland

Abstract:

Chemical changes in groundwater before and after earthquakes have been compared with analyses of drill cuttings taken from a borehole in northern Iceland. Groundwater samples were taken on a weekly basis from September 2008 to June 2015 from a 100 m deep borehole at Hafralækur which is situated in the seismically-active Tjörnes Fracture Zone (TFZ), a transform plate boundary in northern Iceland. Chemical analyses of groundwater samples revealed significant increases in the concentrations of Na, Si, Ca and Al before consecutive M>5 earthquakes in October 2012 and April 2013 (Skelton et al. 2014) and an ongoing decrease in their concentrations after these earthquakes. Chemical analyses of drill cuttings from an adjacent borehole revealed that (1) water inlets are located at the boundaries between lava flows, and (2) Na concentrations in drill cuttings are elevated alongside these water inlets. Further, mineralogical studies of drill cuttings reveal that zeolites are abundant around water inlets and that they occur both filling vesicles and as pseudomorphs of labradorite. Plagioclase is perfectly pseudomorphed by sodic analcime based on which we interpret replacement by an interface-coupled dissolution/precipitation mechanism. Vesicles are filled with calcic chabazite. From these replacement textures, we infer that chemical changes in groundwater before and after the M>5 earthquakes were caused by cation release during constant volume replacement of plagioclase by zeolites coupled with cation uptake during zeolite formation in vesicles.

Skelton, A., M. Andrén, et al. (2014). "Changes in groundwater chemistry before two consecutive earthquakes in Iceland." Nature Geoscience 7(10): 752-756.