Geochemistry of summit fumarole vapors and flanking thermal/mineral waters at Popocatepetl Volcano, Mexico

Abstract

Popocatepetl Volcano is potentially devastating to populations living in the greater Mexico City area. Systematic monitoring of fumarole gases and flanking thermaVmineral springs began in early 1994 after increased fumarolic and seismic activity were noticed in 1991. These investigations had two major objectives: 1) to determine if changes in magmatic conditions beneath Popocatepetl might be reflected by chemical changes in fumarolic discharges and 2) to determine if thermaVminera1 spring waters in the vicinity of Popocatkpetl are geochemically related to or influenced by the magmatic system. This report summarizes results from these two discrete studies.Chemical and isotopic compositions of gases were monitored by placing alkaline traps containing 4N KOH around the crater and by collecting the trap solutions at near-monthly intervals until the volcano erupted on December 21,1994. Absolute abundances of volatile constituents (ppdday of C1, &tal, F, COz, Hg, and As) show general increases during the June-July 1994 and 0ct.-Dec. 1994 time periods. These increases correlated with increased SO2 output measured by correlation spectrometry (COSPEC). The full range of sulfur isotope compositions measured during this study is 1.5 to 6.4%0; the lower values generally occurred in the 0ct.-Dec. samples. These values are very close to that of mantle sulfur (-O%O). The higher values occurred in samples from the June-July period, suggesting possible incorporation of hydrothermal sulfate or underlying Cretaceous evaporitic rocks (roughly 16 to 19%0) into the magma during that time. Carbon isotope compositions measured on the trap solutions were not as useful because of contamination with atmospheric C02. Bulk chemistry and sulfur isotope variations of the volatiles can be explained by injection of mafic magma into a silicic magma chamber beneath the volcano, ascent of magma in the conduit or chamber, and incorporation of small volumes of altered rocks or older strata into the evolving melts.The flanking thermdmineral waters, which were collected periodically from May 1994 until June 1995, show no apparent geochemical changes with time, indicating that increased volcanic activity did not affect these springs. These fluids are relatively low in Si02, B, Br, Cs, Li, and Rb compared with those from known geothermal systems, indicating that Popocatepetl springs are not derived from a high-temperature reservoir. Chemical geothermometry of the waters and feeble gas emissions suggest equilibration temperatures of 160°C. The waters show chemical equilibrium with carbonate and evaporite minerals (calcite and gypsum), which are known to occur in regional Cretaceous strata. Limited stable isotope data on the waters indicate recharge at elevations considerably lower than Popocat6petl summit, whereas tritium contents indicate that the waters have maximum mean residence times of approximately 1000 years.

The crater lake and hydrothermal system of Mount Pinatubo, Philippines: evolution in the decade after eruption

“…Field pH measurements were obtained using pH-sensitive papers. Water and gas samples were analyzed for their chemistry at Los Alamos National Laboratory (LANL), following the procedures of Trujillo et al (1987), Fahlquist and Janik (1992), Werner et al (1997) and Goff et al (2002).…”

Mixing of groundwaters with uncertain end-members: case study in the Tepalcingo-Axochiapan aquifer, Mexico

“…These springs represent one of the end-members of the postulated mixing process in the area. Werner et al (1997) sampled Again the chemical composition of the water is variable in time and it reflects interaction with the underlying carbonate aquifer because the water is saturated with respect to calcite, gypsum, dolomite and fluorite. Since its isotopic composition (δD and δ 18 O) fall on the local meteoric water line, the spring water is mainly of meteoric origin.…”

“…The Piper diagram in Fig. 5 includes published data from Werner et al (1997) and Martin-Del Pozzo et al (2002) for the springs in Atotonilco. These data allow characterizing the temporal variability of the discharge of the limestone aquifer.…”

“…Sulfate and calcium are also considered in the mixing calculations because of the contrasting concentration between the two end members. Mixing computations were conducted first including only chloride, then adding sulfate Werner et al (1997). All data sets have charge-balance errors less than 5% b AT-III (18 samples in 20 months) from Martin-Del Pozzo et al (2002).…”

“…This area presents some interesting hydrogeologic features and also presents some challenges in water-resources management. From the hydrologic point of view, the hydrogeologic setting is composed by two main aquifers, a lower carbonate aquifer underlying an upper volcanic-sedimentary aquifer; in a regional setting, these aquifers may be connected by regional flow systems (Vázquez-Sánchez et al 1989;Werner et al 1997). Even though the interaction between these two aquifers is recognized, it remains to be characterized and quantified.…”