Passive infrared remote sensing evidence for large, intermittent CO 2 emissions at Popocatépetl volcano, Mexico

Abstract

Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatépetl volcano during February 10 to 26, 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively quiet and SO 2 flux averaged 1670±1420 t/day (51 measurements), relatively small for Popocatépetl. Concurrent HCl/SO 2 and HF/SO 2 ratios were 0.17±0.01 and 0.031±0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO 2 in the volcanic plume was quantified using FASCODE in which atmospheric CO 2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic CO 2. Surprisingly, CO 2/SO 2 mass ratios rose dramatically to values as high as 140, about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996. These excursions in high CO 2/SO 2 ratios were short-lived, lasting no longer than about 0.5 to 3.0 h but CO 2 flux occasionally exceeded 100,000 t/day. We estimate that the average CO 2/SO 2 ratio for the period was about 23, yielding an average CO 2 flux of roughly 38,000 t/day. Chemical and petrographic analyses of lava and pumice erupted during explosions on June 30, 1997 and January 1, 1998 show conclusively that Popocatépetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040°C and 5 kbar. In addition, Popocatépetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO 2/SO 2 ratio of 4 for reference, we calculate an average excess CO 2 production of 32,000 t/day for 17 days. This would require assimilation of only 5×10 −4 km 3 of limestone, an amount easily accessible in the 3-km-thick Cretaceous section beneath the volcano. We also examine two scenarios in which excess CO 2 is produced by degassing of subjacent basalt magma, but these explanations seem less plausible to us. Because many other volcanoes are underlain by carbonate sequences, short-duration bursts of CO 2 flux, and increased CO 2/SO 2 ratio, might be observed at other sites, if simultaneous, real-time measurements of major gas species are made.