Passive infrared remote sensing evidence for large, intermittent CO2 emissions at Popocatepetl volcano, Mexico

Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatepetl volcano during February 10 to 26. 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively q uiet and SO2 flux averaged 1670 +/- 1420 t/day (51 measurements). relativel y small for Popocatepetl. Concurrent HCl/SO2 and HF/SO2 ratios were 0.17 +/ - 0.1 and 0.031 +/- 0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO2 in the volcanic plume was quantified u sing FASCODE in which atmospheric CO2 is numerically subtracted from the to tal infrared spectrum to obtain the residual magmatic CO2. Surprisingly, CO 2/SO2 mass ratios rose dramatically to values as high as 140, about 30 time s higher than typical values of 2 to 8 measured from 1994 to 1996. These ex cursions in high CO2/SO2 ratios were short-lived, lasting no longer than ab out 0.5 to 3.0 h but CO2 flux occasionally exceeded 100,000 t/day. We estim ate that the average CO2/SO2 ratio for the period was about 23, yielding an average CO2 flux of roughly 38,000 t/day. Chemical and petrographic analys es of lava and pumice erupted during explosions on June 30, 1997 and Januar y 1, 1998 show conclusively that Popocatepetl produces mixed products forme d by injection of mafic magma into a more silicic chamber at temperatures a nd pressures of roughly 1040 degreesC and 5 kbar. In addition. Popocatepetl eruptive products include xenoliths of metamorphosed carbonate rocks conta ining wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO2/ SO2 ratio of 4 for reference. we calculate an average excess CO2 production of 32,000 t/day for 17 days. This would require assimilation of only 5 x 1 0(-4) km(3) of limestone, an amount easily accessible in the 3-km-thick Cre taceous section beneath the volcano. We also examine two scenarios in which excess CO2 is produced by degassing of subjacent basalt magma, but these e xplanations seem less plausible to us. Because many other Volcanoes are und erlain by carbonate sequences, short-duration bursts of CO2 flux, and incre ased CO2/SO2 ratio, might be observed at other sites, if simultaneous, real -time measurements of major gas species are made. Published by Elsevier Sci ence B.V.