Ž . Chemical Geology 177 2001 157–173 www.elsevier.comrlocaterchemgeo The absence of diffuse degassing at Popocatepetl volcano, ´ Mexico N.R. Varley ) , M.A. Armienta Instituto de Geofısica, Ciudad UniÕersitaria, Coyoacan CP 04360, Mexico ´ ´ Received 13 September 1999; accepted 7 April 2000 Abstract Soil gas or flux measurements have shown that many active volcanoes release significant volumes of magmatic gas Ž 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 Popocatepetl started in December 1994. Since then, frequent ash emissions have ´ been occurring and very large quantities of SO and CO have been measured in the summit plume: up to 60,000 t day y1 of 2 2 SO . From August 1997 until April 1999 over 500 measurements of Rn and CO in soil gas were performed, with extensive 2 2 coverage of the volcano. Surprisingly, no CO of magmatic origin has been detected. Levels of up to 14% have been 2 measured in some locations, however, with d 13 C values of between y17‰ and y25‰, its origin appears to be biogenic. The average RrR value for the He isotopic ratio of samples of soil gas was 0.49 with all samples giving a radiogenic rather a 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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Soil gas; Carbon dioxide; Carbon isotopes; Radon; Helium isotopes; Popocatepetl ´ 1. Introduction Measurements of soil gases have been used exten- sively in the exploration for mineral deposits, and for the location of geological structures, such as faults Ž . Klusman, 1993; Toutain and Baubron, 1999 . Much ) Corresponding author. 1484 Morgan Rd., San Bernardino, CA 92407, USA. Tel.: q 1-909-880-9786; fax: q 1-603-853-5071. Ž . E-mail address: eruption@iname.com N.R. Varley . effort has been directed towards using the variation of soil gases as a predictive precursor to earthquakes Ž . King, 1990; Toutain and Baubron, 1999 . More recently, it has become apparent that passively de- gassing volcanoes can release significant quantities of gas through their flanks as well as from crater fumaroles. Large quantities have been measured in the soil gas at several volcanoes around the world Ž Chiodini et al., 1996; Koepenick et al., 1996; . Heiligmann et al., 1997; Allard et al., 1997 . The principal gas measured has been CO , with rare 2 0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 00 00389-2