Geochemistry of ash leachates during the 1994–1996 activity of Popocatépetl volcano

Increasing fumarolic activity at Popocatépetl volcano has been observed since 1992. On 21 December 1994, a series of eruptions at Popocatépetl volcano produced ash emissions that reached the city of Puebla located to the east of the volcano. Eruptive activity declined sharply from June 1995 until 5 March 1996 when ash emissions and fumarole flux increased to levels similar to those of December, 1994. Intermittent ash production has continued to 1997. Ash was sampled at more than 80 different locations around the volcano during the various eruptions. Gas produced 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 eruptions from December 1994 until 28 November 1996, were analyzed by potentiometry with selective electrodes for Cl− and F− and by ion chromatography for SO42−. Minor cations (Co2+, Pb2+, Zn2+, Cu2+, Mn2+, Sb2+, Ti4+, Cd2+, Tl3+) were determined in some samples by ICP-MS. The highest concentrations of Cl− and SO42− were obtained for the 21 December 1994 ash 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 observed, but with particular trends in each case. The composition of the ash leachates suggests that the two Popocatépetl eruptions in 1994 and 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 outpouring of lava in the bottom of the crater on 20 March 1996. The concentration trends for SO42−, Cl− and F− suggest that during the 1996 activity, the system attained higher temperatures than in 1994–1995.