Transport of mantle volatiles through the crust traced by seismically released fluids: a natural experiment in the earthquake swarm area Vogtland/NW Bohemia, Central Europe

Abstract

Vogtland, which is located on the Czech–German border near the crossing of the Eger Rift and the Mariánské Lázně fault zone, is characterised by CO2-rich mineral springs fed by a magmatic source and swarm earthquake activity. The free gas of the Eisenquelle mineral spring was studied weekly from April 1994 to June 1996, a period which included a quake swarm on December 4 and 5, 1994, comprising approximately 500 individual events Mlocal≤2.2. Co-seismic shifts in He content after about a month as well as in the δ13C of CO2 and 3He/4He after about 220days were resolved from seasonal background variations. The He concentration shift is assumed to originate near the discharge, whereas the isotopic shifts are probably caused by the addition of crustal fluids released by crushing rocks within the hypocentre. From the distance of 14km between the hypocentre to the mineral spring, the transport velocity of the released crustal fluids was determined to be at least 50m/day. Such a high speed indicates gaseous fluid transport along channel-like conduits, rather than being predominantly controlled by dissolution in water.Using a simple mass balance it is shown that the amount of crustal fluids potentially released by seismic activity strongly exceeds that required for the observed shifts in 3He/4He and δ13C. This fact along with the coincidence of both shifts supports the hypothesis that magmatic fluids occurring in mineral waters of the Eisenquelle environs ‘flush’ the hypocentral crustal block by means of advectional transport.