CO2 and He degassing at El Chichn volcano, Chiapas, Mexico: gas flux, origin and relationship with local and regional tectonics

During 2007-2008, three CO2 flux surveys were performed on El Chichn volcanic lake, Chiapas, Mexico, with an additional survey in April 2008 covering the entire crater floor (including the lake). The mean CO2 flux calculated by sequential Gaussian simulation from the lake was 1,190 (March 2007), 730 (December 2007) and 1,134 g m(-2) day(-1) (April 2008) with total emission rates of 164 +/- 9.5 (March 2007), 59 +/- 2.5 (December 2007) and 109 +/- 6.6 t day(-1) (April 2008). The mean CO2 flux estimated from the entire crater floor area was 1,102 g m(-2) day(-1) for April 2008 with a total emission rate of 144 +/- 5.9 t day(-1). Significant change in CO2 flux was not detected during the period of survey, and the mapping of the CO2 flux highlighted lineaments reflecting the main local and regional tectonic patterns. The He-3/He-4 ratio (as high as 8.1 R (A)) for gases in the El Chichn crater is generally higher than those observed at the neighbouring Transmexican Volcanic Belt and the Central American Volcanic Arc. The CO2/He-3 ratios for the high He-3/He-4 gases tend to have the MORB-like values (1.41 Au 10(9)), and the CO2/He-3 ratios for the lower He-3/He-4 gases fall within the range for the arc-type gases. The high He-3/He-4 ratios, the MORB-like CO2/He-3 ratios for the high He-3/He-4 gases and high proportion of MORB-CO2 (M = 25 +/- 15%) at El Chichn indicate a greater depth for the generation of magma when compared to typical arc volcanoes.