The absence of diffuse degassing at Popocatépetl volcano, Mexico

Soil gas or flux measurements have shown that many active volcanoes release significant volumes of magmatic gas through their flanks. This type of measurement represents a useful and safer method (compared with direct fumarole measurements) for monitoring an active volcano. However, this study shows that not all volcanoes will have a significant diffuse component, even if substantial quantities of gas are released from the crater. The current phase of volcanic activity at Popocatépetl started in December 1994. Since then, frequent ash emissions have been occurring and very large quantities of SO 2 and CO 2 have been measured in the summit plume: up to 60,000 t day −1 of SO 2. From August 1997 until April 1999 over 500 measurements of Rn and CO 2 in soil gas were performed, with extensive coverage of the volcano. Surprisingly, no CO 2 of magmatic origin has been detected. Levels of up to 14% have been measured in some locations, however, with δ 13C values of between −17‰ and −25‰, its origin appears to be biogenic. The average R/ R a value for the He isotopic ratio of samples of soil gas was 0.49 with all samples giving a radiogenic rather than magmatic signature. Some large ravines in the edifice are thought to mark the location of faults. Large displacements have been measured, but only low concentrations of soil gases were recorded. There is no evidence of a geothermal system, which is likely to be connected to the absence of a diffuse gas component.