GEOCHEMICAL SURVEILLANCE OF MAGMATIC VOLATILES AT POPOCATEPETL VOLCANO, MEXICO

Surveillance of Popocatepetl volcanic plume geochemistry and SO2 flux began in early 1994 after fumarolic and seismic activity increased sig nificantly during 1993, Volatile traps placed around the summit were c ollected at near-monthly intervals until the volcano erupted on Decemb er 21, 1994, Additional trap samples were obtained in early 1996 befor e the volcano erupted again, emplacing a small dacite dome in the summ it crater. Abundances of volatile constituents (ppm/day of CL, S-total , F, CO2, Hg, and As) varied, but most constituents were relatively hi gh in early and late 1994, However, ratios of these constituents to Cl were highest in mid-1994, delta(34)S-S-total in trap solutions ranged from 1.5 parts per thousand to 6.4 parts per thousand; lowest values generally occurred during late 1994, delta(13)C-CO2 of trap solutions were greatly contaminated with atmospheric CO2 and affected by absorpt ion kinetics. When trap data are combined with SO2 flux measurements m ade through November 1996, Popocatepetl released about 3.9 Mt SO2, 16 Mt CO2, 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H2O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTI R) commonly exceed human recommended exposure limits and may constitut e a potential health hazard. Volatile geochemistry combined with petro logic observations and melt-inclusion studies show that mafic magma in jection into a preexisting silicic chamber has accompanied renewed vol canism at Popocatepetl. Minor assimilation of Cretaceous wall rocks pr obably occurred in mid-1994.