GEOCHEMISTRY OF ASH LEACHATES DURING THE 1994-1996 ACTIVITY OF POPOCATEPETL VOLCANO

Increasing fumarolic activity at Popocatepetl volcano has been observe d since 1992. On 21 December 1994, a series of eruptions at Popocatepe tl volcano produced ash emissions that reached the city of Puebla loca ted to the east of the volcano. Eruptive activity declined sharply fro m June 1995 until 5 March 1996 when ash emissions and fumarole Bur inc reased to levels similar to those of December, 1994. Intermittent ash production has continued to 1997, Ash was sampled at more than 80 diff erent locations around the volcano during the various eruptions. Gas p roduced during an eruption may be scavenged by the ash and leaching of the ash with water allows determination of the concentration of ions adsorbed from the volcanic gases. The leachates obtained from eruption s from December 1994 until 28 November 1996, were analyzed by potentio metry with selective electrodes for Cl- and F- and by ion chromatograp hy for SO42-. Minor calions (Co2+, Pb2+, Zn2+, Cu2+, Mn2+, Sb2+, Ti4+, Cd2+, Tl3+) were determined in some samples by ICP-MS. The highest co ncentrations of Cl- and SO42- were obtained for the 21 December 1994 a sh at the start of the eruptions with 19 550 ppm SO42- and 1028 ppm Cl - and for the emission which occurred on 5 March 1996, with 21 775 ppm SO42- and 1250 ppm Cl-. At both times a concentration decrease was ob served, but with particular trends in each case. The composition of th e ash leachates suggests that the two Popocatepetl eruptions in 1994 a nd 1996 began with phreatic and magmatic components. The increase in F - and the decrease in the Cl/F ratio may indicate a heating up of the volcanic system at the beginning of March, 1996, one week before the o utpouring of lava in the bottom of the crater on 20 March 1996. The co ncentration trends for SO42-, Cl- and F- suggest that during the 1996 activity, the system attained higher temperatures than in 1994-1995. ( C) 1998 Elsevier Science Ltd. All rights reserved.